Add more documentation about ways to configure language servers and rust-analyzer (#29932)

Release Notes:

- N/A

docs/src/languages/rust.md

@@ -119,6 +119,39 @@ If you are using `rustup` and you can find a list of available target triples (`
 rustup target list --installed
 ```
 
+## LSP tasks
+
+Zed provides tasks using tree-sitter, but rust-analyzer has an LSP extension method for querying file-related tasks via LSP.
+This is enabled by default and can be configured as
+
+```json
+"lsp": {
+  "rust-analyzer": {
+    enable_lsp_tasks": true,
+  }
+}
+```
+
+## Manual Cargo Diagnostics fetch
+
+By default, rust-analyzer has `checkOnSave: true` enabled, which causes every buffer save to trigger a `cargo check --workspace --all-targets` command.
+For lager projects this might introduce excessive wait times, so a more fine-grained triggering could be enabled by altering the
+
+```json
+"diagnostics": {
+  "cargo": {
+    // When enabled, Zed disables rust-analyzer's check on save and starts to query
+    // Cargo diagnostics separately.
+    "fetch_cargo_diagnostics": false
+  }
+}
+```
+
+default settings.
+
+This will stop rust-analyzer from running `cargo check ...` on save, yet still allow to run
+`editor: run/clear/cancel flycheck` commands in Rust files to refresh cargo diagnostics; the project diagnostics editor will also refresh cargo diagnostics with `editor: run flycheck` command when the setting is enabled.
+
 ## More server configuration
 
 <!--