gpui: Fix intra rustdoc links (#37320)

The only warnings remaining are links to private modules/items, but I
lack knowledge to work out if the referenced modules/items should be
made public, or if the links should be rewritten into exposed
traits/items.

Links to associated items such as trait implementations have to be
written using full markdown format such as:

... [[ `App::update_global` ]](( BorrowAppContext::update_global ))

This is due to https://github.com/rust-lang/rust/issues/74563 which
sadly prohibits fully-qualified syntax:

... [[ `<App as BorrowAppContext>::update_global` ]]

Release Notes:

- N/A

Probably related to https://github.com/zed-industries/zed/pull/37072

crates/gpui/src/app/async_context.rs

@@ -218,7 +218,7 @@ impl AsyncApp {
         Some(read(app.try_global()?, &app))
     }
 
-    /// A convenience method for [App::update_global]
+    /// A convenience method for [`App::update_global`](BorrowAppContext::update_global)
     /// for updating the global state of the specified type.
     pub fn update_global<G: Global, R>(
         &self,
@@ -293,7 +293,7 @@ impl AsyncWindowContext {
             .update(self, |_, window, cx| read(cx.global(), window, cx))
     }
 
-    /// A convenience method for [`App::update_global`].
+    /// A convenience method for [`App::update_global`](BorrowAppContext::update_global).
     /// for updating the global state of the specified type.
     pub fn update_global<G, R>(
         &mut self,