Show descriptions for `:alpha` items in GraphQL docs

What does this MR do and why?

Previously the original descriptions of "alpha" items were not displayed in the generated GraphQL docs. This was because marking items as "alpha" leverages our deprecation code, and for deprecated items we only display the deprecation notice and not the original item description.

For alpha fields, we now display the warning of its instability but also the original description.

Screenshots or screen recordings

Before	After

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

spec/lib/gitlab/graphql/deprecation_spec.rb

@@ -241,13 +241,13 @@
   describe '#alpha?' do
     let(:options) { { milestone: '10.10', reason: reason } }
 
-    context 'when when `reason` is `:alpha`' do
+    context 'when `reason` is `:alpha`' do
       let(:reason) { described_class::REASON_ALPHA }
 
       it { is_expected.to be_alpha }
     end
 
-    context 'when when `reason` is not `:alpha`' do
+    context 'when `reason` is not `:alpha`' do
       let(:reason) { described_class::REASON_RENAMED }
 
       it { is_expected.not_to be_alpha }