Update doc comments with tabs.file_icons default (#17629)

The diff only contains doc comments changes, however I expect this also
fixes generating JSON Schema which generated by
[schemars](092dc17ae4/docs/examples/6-doc_comments.md).

This default value is actually true at first.

1818fef32f

However, it was changed in the following commit.

bf7e474bbc

Closes #17628

Release Notes:

- N/A

crates/workspace/src/item.rs

@@ -79,7 +79,7 @@ pub struct ItemSettingsContent {
     close_position: Option<ClosePosition>,
     /// Whether to show the file icon for a tab.
     ///
-    /// Default: true
+    /// Default: false
     file_icons: Option<bool>,
 }