time: rewrite the documentation for layout strings

People continue to be confused by how these work. Address that by some
rejiggering.

Introduce a constant called Layout that both defines the time and
provides a reference point for Parse and Format to refer to. We can
then delete much redundancy, especially for Format's comments, but
Parse tightens a bit too.

Then change the way the concept of the layout string is introduced,
and provide a clearer catalog of what its elements are.

Fixes #38871

Change-Id: Ib967ae70c7d5798a97b865cdda1fda4daed8a99a
Reviewed-on: https://go-review.googlesource.com/c/go/+/320252


Trust: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>