spec: combine section on type parameters and type parameter lists

This change moves the relevant prose of the section on type parameters
into the section on type parameter lists and eliminates the former.

With this change, the section on types now exclusively describes all
Go composite types.

User-defined named types (defined types and type parameters) are
described with their declarations.

Change-Id: I3e421cd236e8801d31a4a81ff1e5ec9933e3ed20
Reviewed-on: https://go-review.googlesource.com/c/go/+/385037


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>