DEV: Update api docs to update username and to get post replies (#15881)

Added api docs for:

 - update username
 - get post replies

spec/requests/api/users_spec.rb

@@ -176,7 +176,6 @@ describe 'users' do
   end
 
   path '/u/{username}/preferences/email.json' do
-
     put 'Update email' do
       tags 'Users'
       operationId 'updateEmail'
@@ -201,7 +200,33 @@ describe 'users' do
         end
       end
     end
+  end
 
+  path '/u/{username}/preferences/username.json' do
+    put 'Update username' do
+      tags 'Users'
+      operationId 'updateUsername'
+      consumes 'application/json'
+      expected_request_schema = load_spec_schema('user_update_username_request')
+
+      parameter name: :username, in: :path, type: :string, required: true
+      parameter name: :params, in: :body, schema: expected_request_schema
+
+      produces 'application/json'
+      response '200', 'username updated' do
+
+        let(:user) { Fabricate(:user) }
+        let(:username) { user.username }
+        let(:params) { { 'new_username' => "#{user.username}1" } }
+
+        expected_response_schema = nil
+
+        it_behaves_like "a JSON endpoint", 200 do
+          let(:expected_response_schema) { expected_response_schema }
+          let(:expected_request_schema) { expected_request_schema }
+        end
+      end
+    end
   end
 
   path '/directory_items.json' do