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 <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>