Document git.gutter_debounce setting (#22663)

Closes #22588 by providing documentation to git.gutter_debounce setting

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/project/src/project_settings.rs

@@ -85,6 +85,9 @@ pub struct GitSettings {
     ///
     /// Default: tracked_files
     pub git_gutter: Option<GitGutterSetting>,
+    /// Sets the debounce threshold (in milliseconds) after which changes are reflected in the git gutter.
+    ///
+    /// Default: null
     pub gutter_debounce: Option<u64>,
     /// Whether or not to show git blame data inline in
     /// the currently focused line.