docs: Describe how preview tabs actually behave (#63875)

# Objective

- Fixes #33819
- The `preview_tabs` documentation says preview tabs "close
automatically when you switch to another file unless you explicitly pin
them". They do not close. Moving to another file leaves the preview tab
open — either still as the preview tab, or converted into a regular one.
Four people ran into this on the issue over the past year.
- The same section then lists the ways to turn a preview tab into a
regular tab, and two of them are missing, including the pinning that the
sentence above uses as its counter-example.

## Solution

Two corrections to the `Preview tabs` section of
`docs/src/reference/all-settings.md`. No behaviour was touched.

**The description.** A pane holds a single `preview_item_id`, so there
is at most one preview tab at a time. What happens to it when you move
to another file depends on the path:

- Opening a not-yet-open file in preview mode from the project panel or
file finder goes through `Pane::open_item`, which calls
`close_current_preview_item` — that really does close the old tab
(`remove_item`) and hands back its index, and the new file is inserted
there.
- Code navigation with the default
`enable_keep_preview_on_code_navigation: false` instead calls
`unpreview_item_if_preview` on the old item *before*
`replace_preview_item_id`, so the previous file stays open as a regular
tab.

A third path, the file finder with the default
`enable_preview_from_file_finder: false`, opens the new file as a
regular tab and leaves the existing preview tab alone entirely.

Rather than spell out every path in a settings description — the
`enable_preview_*` settings immediately below already cover them — the
new wording sticks to what holds in all of them: a pane keeps at most
one preview tab and opening another file in preview mode takes over that
slot, and switching to a file that is already open does not close the
preview tab. That second half is the case the issue actually reports.

**The list.** Two more ways to convert a preview tab into a regular tab:

- Pinning it — `change_tab_pin_state` calls `unpreview_item_if_preview`
on the `Pin` branch.
- `pane::TogglePreviewTab` — the action dedicated to exactly this,
registered whenever preview tabs are enabled.

## Testing

- Built a debug Zed and walked through the steps from the issue on a
scratch project of `a.txt` / `b.txt` / `c.txt`: with `a.txt` open as a
regular tab and `b.txt` opened as a preview tab, clicking back to
`a.txt` leaves the `b.txt` preview tab open, which is what the current
wording says should not happen. Single-clicking `c.txt` takes over the
same slot rather than adding a second preview tab, and pinning the
preview tab drops the italics.
- Ran the docs steps from `run_tests.yml` locally: `zed
--dump-all-actions > crates/docs_preprocessor/actions.json`, then
`mdbook build ./docs` on mdbook 0.4.37 to match CI. Exit 0 with no
preprocessor errors, and both new `{#action ...}` references resolve —
the rendered page reads "pane: toggle pin tab" and "pane: toggle preview
tab" in the same style as the existing "project panel: open permanent".
- `cargo test -p workspace --lib` — 272 passed, 0 failed, including
`test_close_other_items_unpreviews_active_item`,
`test_reopening_closed_item_after_unpreview`, and the proptest that
opens random paths with random `allow_preview` and asserts a pane never
holds more than one preview item.

## Notes for review

- Several people on the issue would rather the *behaviour* changed, so
that preview tabs close on any tab switch the way the docs currently
describe. I left that alone deliberately: the issue is typed `Docs`, so
this PR only makes the prose match the implementation. Happy to open a
discussion separately if it is worth pursuing.
- Saving a file also unpreviews it (`Pane::save_item`), but I left it
off the list as redundant — you have to edit a file before you can save
it, and editing is already there. Say the word if you would rather it
were explicit.
- The comment on `preview_tabs.enabled` in
`assets/settings/default.json` words this as "close automatically when
you open another preview tab". That is right for the project-panel path
and carries the same code-navigation caveat as above; I left it alone to
keep this diff to one file, and am happy to align it in a follow-up.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments — N/A
- [x] The content adheres to Zed's UI standards — N/A, docs only
- [x] Tests cover the new/changed behavior — N/A, no behaviour change;
the docs build is verified locally
- [x] Performance impact has been considered and is acceptable — N/A

---

Release Notes:

- N/A

docs/src/reference/all-settings.md

@@ -3575,7 +3575,7 @@ Examples:
 ## Preview tabs
 
 - Description:
-  Preview tabs allow you to open files in preview mode, where they close automatically when you switch to another file unless you explicitly pin them. This is useful for quickly viewing files without cluttering your workspace. Preview tabs display their file names in italics. \
+  Preview tabs allow you to open files in preview mode. A pane keeps at most one preview tab at a time, so opening another file in preview mode takes over that slot. Switching to a file that is already open does not close the preview tab. This is useful for quickly viewing files without cluttering your workspace. Preview tabs display their file names in italics. \
   There are several ways to convert a preview tab into a regular tab:
 
   - Double-clicking on the file
@@ -3583,6 +3583,8 @@ Examples:
   - Using the {#action project_panel::OpenPermanent} action
   - Editing the file
   - Dragging the file to a different pane
+  - Pinning the tab with the {#action pane::TogglePinTab} action
+  - Using the {#action pane::TogglePreviewTab} action
 
 - Setting: `preview_tabs`
 - Default: