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

Release Notes:

- N/A

docs/src/configuring-languages.md

@@ -182,6 +182,63 @@ Here's how you would structure these settings in Zed's `settings.json`:
 }
 ```
 
+#### Possible configuration options
+
+Depending on how a particular language server is implemented, they may depend on different configuration options, both specified in the LSP.
+
+- [initializationOptions](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#version_3_17_0)
+
+Sent once during language server startup, requires server's restart to reapply changes.
+
+For example, rust-analyzer and clangd rely on this way of configuring only.
+
+```json
+  "lsp": {
+    "rust-analyzer": {
+      "initialization_options": {
+        "checkOnSave": false
+      }
+    }
+  }
+```
+
+- [Configuration Request](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_configuration)
+
+May be queried by the server multiple times.
+Most of the servers would rely on this way of configuring only.
+
+```json
+"lsp": {
+  "tailwindcss-language-server": {
+    "settings": {
+      "tailwindCSS": {
+        "emmetCompletions": true,
+      },
+    }
+  }
+}
+```
+
+Apart of the LSP-related server configuration options, certain servers in Zed allow configuring the way binary is launched by Zed.
+
+Languages mention in the documentation, whether they support it or not and their defaults for the configuration values:
+
+```json
+  "languages": {
+    "Markdown": {
+      "binary": {
+        // Whether to fetch the binary from the internet, or attempt to find locally.
+        "ignore_system_version": false,
+        "path": "/path/to/langserver/bin",
+        "arguments": ["--option", "value"],
+        "env": {
+          "FOO": "BAR"
+        }
+      }
+    }
+  }
+```
+
 ### Enabling or Disabling Language Servers
 
 You can toggle language server support globally or per-language:

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
 
 <!--