Deprecate completions from plugins api.

Change-Id: I92737e2df4736672c93fc9a08eb7c90a41192afb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397882
Commit-Queue: Keerti Parthasarathy <keertip@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analyzer_plugin/doc/api.html

@@ -673,6 +673,7 @@ a:focus, a:hover {
         </p>
       </dd></dl></dd></dl>
 <h2 class="domain"><a name="domain_completion">completion domain</a></h2>
+  <p>Deprecated - no longer supported.</p>
   <p>
     The code completion domain contains API's related to getting code completion
     suggestions.
@@ -694,6 +695,7 @@ a:focus, a:hover {
     "<b>results</b>": List&lt;<a href="#type_CompletionSuggestion">CompletionSuggestion</a>&gt;
   }
 }</pre></div>
+    <p>Deprecated - no longer supported.</p>
     <p>
       Used to request that completion suggestions for the given offset in the
       given file be returned.

pkg/analyzer_plugin/test/integration/support/integration_test_methods.dart

@@ -433,6 +433,8 @@ abstract class IntegrationTestMixin {
   /// Stream controller for [onAnalysisOutline].
   late StreamController<AnalysisOutlineParams> _onAnalysisOutline;
 
+  /// Deprecated - no longer supported.
+  ///
   /// Used to request that completion suggestions for the given offset in the
   /// given file be returned.
   ///

pkg/analyzer_plugin/tool/spec/plugin_spec.html

@@ -542,11 +542,13 @@
   </notification>
 </domain>
 <domain name="completion">
+  <p>Deprecated - no longer supported.</p>
   <p>
     The code completion domain contains API's related to getting code completion
     suggestions.
   </p>
   <request method="getSuggestions">
+    <p>Deprecated - no longer supported.</p>
     <p>
       Used to request that completion suggestions for the given offset in the
       given file be returned.