extension_api: Improve documentation for make_file_executable (#48198)

Extension authors frequently guard this method in their extensions
although this is not necessary.

Thus, this PR updates the documentation of `make_file_executable` with a
brief mention to indicate that this is not needed.

Release Notes:

- N/A

crates/extension_api/wit/since_v0.8.0/extension.wit

@@ -57,6 +57,8 @@ world extension {
     import download-file: func(url: string, file-path: string, file-type: downloaded-file-type) -> result<_, string>;
 
     /// Makes the file at the given path executable.
+    ///
+    /// This is a no-op on Windows.
     import make-file-executable: func(filepath: string) -> result<_, string>;
 
     /// Updates the installation status for the given language server.