Do not show completion documentation if disabled (#7372)

This fixes #7348 by not rendering completions if they are disabled in
the settings. Previously we only checked that setting when starting or
not starting background threads to fetch documentation. But in case we
already have documentation, this stops it from being rendered.

Release Notes:

- Fixed documentation in completions showing up even when disabled via
`show_completion_documentation`
([#7348](https://github.com/zed-industries/zed/issues/7348))

crates/editor/src/editor.rs

@@ -852,7 +852,7 @@ impl CompletionsMenu {
         let selected_item = self.selected_item;
         let style = style.clone();
 
-        let multiline_docs = {
+        let multiline_docs = if show_completion_documentation {
             let mat = &self.matches[selected_item];
             let multiline_docs = match &self.completions.read()[mat.candidate_id].documentation {
                 Some(Documentation::MultiLinePlainText(text)) => {
@@ -883,6 +883,8 @@ impl CompletionsMenu {
                     // because that would move the cursor.
                     .on_mouse_down(MouseButton::Left, |_, cx| cx.stop_propagation())
             })
+        } else {
+            None
         };
 
         let list = uniform_list(