Clean up gpui docs (#4240)

This PR does some cleanup of the `gpui` docs, making sure we're linking
things appropriately and following good Rust documentation style.

Release Notes:

- N/A

crates/gpui/src/shared_string.rs

@@ -4,7 +4,7 @@ use std::{borrow::Borrow, sync::Arc};
 use util::arc_cow::ArcCow;
 
 /// A shared string is an immutable string that can be cheaply cloned in GPUI
-/// tasks. Essentially an abstraction over an Arc<str> and &'static str,
+/// tasks. Essentially an abstraction over an `Arc<str>` and `&'static str`,
 #[derive(Deref, DerefMut, Eq, PartialEq, Hash, Clone)]
 pub struct SharedString(ArcCow<'static, str>);