[clang-tools-extra] [clangd] Enable query-based custom checks (PR #216336)

[Baranov Victor via cfe-commits]

================
@@ -63,6 +63,25 @@ Example
     main(); // warning: call to main function. [custom-call-main-function]
   }
 
+Using with clangd
+=================
+
+To enable query-based custom checks in clangd, add the following to the clangd
+configuration:
+
+.. code-block:: yaml
+
+  Diagnostics:
+    ClangTidy:
+      ExperimentalCustomChecks: true
+      FastCheckFilter: Loose
+
+The custom-check definitions and check selection remain part of the clang-tidy
+configuration. ``ExperimentalCustomChecks`` does not override clangd's
----------------
vbvictor wrote:

```suggestion
configuration. `ExperimentalCustomChecks` does not override clangd's
```

https://github.com/llvm/llvm-project/pull/216336