Move diagnostics to `diagnostics` domain (#24931).

pkg/analysis_server/tool/spec/spec_input.html

Index: pkg/analysis_server/tool/spec/spec_input.html
diff --git a/pkg/analysis_server/tool/spec/spec_input.html b/pkg/analysis_server/tool/spec/spec_input.html
index 45aae246d9cf97cce44f49c35b5b3d975cea7d0f..d519a21fea2017819aa19ebff80171ff4d9d7f1f 100644
--- a/pkg/analysis_server/tool/spec/spec_input.html
+++ b/pkg/analysis_server/tool/spec/spec_input.html
@@ -1959,12 +1959,12 @@
         </params>
       </notification>
     </domain>
-    <domain name="experimental" experimental="true">
+    <domain name="diagnostic" experimental="true">
       <p>
-        The experimental domain contains experimental server APIs.  Experimental
-        APIs are not considered to be part of the analysis server's API, docs are
-        not generated for them and they are subject to change without notice.
-        Because the experimental domain is not part of the analysis server's API,
+        The diagnostic domain contains experimental server diagnostics APIs. Note
+        that as an experimental API, docs are not generated for this domain and it
+        is subject subject to change without notice.
+        Because this experimental domain is not part of the analysis server's API,
         the version number of the API will not be updated when changes are made.
         Caveat emptor!
       </p>