Improve extension API documentation (#10322)

This PR adds some more documentation to symbols exported from the
`zed_extension_api` crate.

Release Notes:

- N/A

crates/extension_api/src/extension_api.rs

@@ -1,5 +1,6 @@
 //! The Zed Rust Extension API allows you write extensions for [Zed](https://zed.dev/) in Rust.
 
+/// Provides access to Zed settings.
 pub mod settings;
 
 use core::fmt;