cmd/compile/internal/types2: mark gotypesalias as undocumented
CL 541737 added gotypesalias to control whether Alias types are used.
This setting is meant to use by end users through go/types. However,
types2 also uses it, but it's an internal package, causing bootstrap
failed because of unknown setting.
Marking the setting as undocumented in types2 fixes the problem.
Fixes #64106
Change-Id: If51a63cb7a21d9411cd9cf81bca2530c476d22f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/542135
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