Add enumconstraint_CSharpKeyword

[TheSench]

Summary

This PR adds an f1_keyword for the generic where clause context of the enum keyword. This allows the F1 help to route to this usage of the keyword, rather than always routing to the class declaration page.

Fixes part of #20799, Roslyn changes under dotnet/roslyn#TBD.

[TheSench]

This is stemming from this part of #20799:

enum: destination describes declaring an enum.

• declare an enum type
• generic constraint where the base type must be an enum (derived from System.Enum)

Is where T : enum now valid, or does it have to be where T : System.Enum? If the new syntax is valid, then this PR requires more work to actually update the document, versus just adding an F1 keyword.

[BillWagner]

Thanks @TheSench

This LGTM, and I'll now.