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

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