go/types, types2: clarify documentation with respect to generic types

go/types, types2: clarify documentation with respect to generic types

Address several areas where documentation was inaccurate or unclear
regarding generic types. Also prefer the use of the word 'generic' over
'parameterized', and add additional documentation for the use of
SetConstraint.

For #49593

Change-Id: Iccac60d1b3e2c45a57a3d03b3c10984293af57dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/391154
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>