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/wit/since_v0.0.6/settings.rs

@@ -1,20 +1,29 @@
 use serde::{Deserialize, Serialize};
 use std::num::NonZeroU32;
 
+/// The settings for a particular language.
 #[derive(Debug, Serialize, Deserialize)]
 pub struct LanguageSettings {
+    /// How many columns a tab should occupy.
     pub tab_size: NonZeroU32,
 }
 
+/// The settings for a particular language server.
 #[derive(Default, Debug, Serialize, Deserialize)]
 pub struct LspSettings {
+    /// The settings for the language server binary.
     pub binary: Option<BinarySettings>,
+    /// The initialization options to pass to the language server.
     pub initialization_options: Option<serde_json::Value>,
+    /// The settings to pass to language server.
     pub settings: Option<serde_json::Value>,
 }
 
+/// The settings for a language server binary.
 #[derive(Debug, Serialize, Deserialize)]
 pub struct BinarySettings {
+    /// The path to the binary.
     pub path: Option<String>,
+    /// The arguments to pass to the binary.
     pub arguments: Option<Vec<String>>,
 }