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;

crates/extension_api/src/settings.rs

@@ -6,6 +6,7 @@ use serde_json;
 pub use types::*;
 
 impl LanguageSettings {
+    /// Returns the [`LanguageSettings`] for the given language.
     pub fn for_worktree(language: Option<&str>, worktree: &Worktree) -> Result<Self> {
         let location = SettingsLocation {
             worktree_id: worktree.id(),
@@ -18,6 +19,7 @@ impl LanguageSettings {
 }
 
 impl LspSettings {
+    /// Returns the [`LspSettings`] for the given language server.
     pub fn for_worktree(language_server_name: &str, worktree: &Worktree) -> Result<Self> {
         let location = SettingsLocation {
             worktree_id: worktree.id(),