spec: adjust representability rules for type parameters
spec: adjust representability rules for type parameters

Change-Id: I4423a059527066c4418c195911f8184dfd3f5a15
Reviewed-on: https://go-review.googlesource.com/c/go/+/365914
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go