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

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

doc/development/api_graphql_styleguide.md

@@ -102,7 +102,7 @@ Two scenarios exist where schema items are exempt from the deprecation process,
 and can be removed or changed at any time without notice. These are schema items that either:
 
 - Use the [`feature_flag` property](#feature_flag-property) _and_ the flag is disabled by default.
-- Are [marked as alpha](#marking-schema-items-as-alpha).
+- Are [marked as alpha](#mark-schema-items-as-alpha).
 
 ## Global IDs
 
@@ -539,7 +539,7 @@ return value of the field. This can be done in the resolver, in the
 type, or even in a model method, depending on your preference and
 situation.
 
-Consider also [marking the field as Alpha](#marking-schema-items-as-alpha)
+Consider also [marking the field as Alpha](#mark-schema-items-as-alpha)
 while the value of the field can be toggled. You can
 [change or remove Alpha fields at any time](#breaking-change-exemptions) without needing to deprecate them.
 This also signals to consumers of the public GraphQL API that the field is not
@@ -586,7 +586,7 @@ To deprecate a schema item in GraphQL:
 See also:
 
 - [Aliasing and deprecating mutations](#aliasing-and-deprecating-mutations).
-- [Marking schema items as Alpha](#marking-schema-items-as-alpha).
+- [Marking schema items as Alpha](#mark-schema-items-as-alpha).
 - [How to filter Kibana for queries that used deprecated fields](graphql_guide/monitoring.md#queries-that-used-a-deprecated-field).
 
 ### Create a deprecation issue