docs: Add note about conflicting global macOS shortcut (#45186)

This is already noted in our `default-macos.json`, but was never
surfaced in our docs for some reason. A user noted their LSP completions
were not working because they were not aware of the conflicting global
shortcut.

Ref:
https://github.com/zed-industries/zed/issues/44970#issuecomment-3664118523

Release Notes:

- N/A

docs/src/completions.md

@@ -15,6 +15,10 @@ When there is an appropriate language server available, Zed will provide complet
 
 You can manually trigger completions with `ctrl-space` or by triggering the `editor::ShowCompletions` action from the command palette.
 
+> Note: Using `ctrl-space` in Zed requires disabling the macOS global shortcut.
+> Open **System Settings** > **Keyboard** > **Keyboard Shortcut**s >
+> **Input Sources** and uncheck **Select the previous input source**.
+
 For more information, see:
 
 - [Configuring Supported Languages](./configuring-languages.md)