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/collab/src/db/queries/notifications.rs

@@ -66,7 +66,7 @@ impl Database {
         .await
     }
 
-    /// Create a notification. If `avoid_duplicates` is set to true, then avoid
+    /// Creates a notification. If `avoid_duplicates` is set to true, then avoid
     /// creating a new notification if the given recipient already has an
     /// unread notification with the given kind and entity id.
     pub async fn create_notification(