[clang] Document the [[clang::annotate]] attribute (PR #203303)

[Erich Keane via cfe-commits]

================
@@ -9145,6 +9145,33 @@ point."
 }];
 }
 
+def AnnotateDocs : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+This attribute is used to add annotations to declarations or statements,
----------------
erichkeane wrote:

```suggestion
The `annotate` attribute is used to add annotations to declarations or statements,
```

https://github.com/llvm/llvm-project/pull/203303