Update tenses of doc comment summary lines (#4161)

This PR updates the tenses used by the summary line of doc comments to
match the [Rust API documentation
conventions](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#summary-sentence).

Specifically:

> The summary line should be written in third person singular present
indicative form. Basically, this means write ‘Returns’ instead of
‘Return’.

I'm sure there are plenty occurrences that I missed.

Release Notes:

- N/A

crates/gpui/src/app/model_context.rs

@@ -189,7 +189,7 @@ impl<'a, T: 'static> ModelContext<'a, T> {
         }
     }
 
-    /// Update the given global
+    /// Updates the given global
     pub fn update_global<G, R>(&mut self, f: impl FnOnce(&mut G, &mut Self) -> R) -> R
     where
         G: 'static,