spec: clarify type term restriction for type parameters
spec: clarify type term restriction for type parameters

Be clear that the type of a term (not the term itself, which may
be of the form ~P) cannot be a type parameter.

For #50420.

Change-Id: I388d57be0618393d7ebe2c74ec04c1ebe3f33f7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/396915
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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