spec: document that type inference doesn't apply to generic types

Type inference for types was always a "nice to have" feature.
Given the under-appreciated complexity of making it work in all
cases, and the fact that we don't have a good understanding of
how it might affect readability of generic code, require explicit
type arguments for generic types.

This matches the current implementation.

Change-Id: Ie7ff6293d3fbea92ddc54c46285a4cabece7fe01
Reviewed-on: https://go-review.googlesource.com/c/go/+/390577


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