Update the regtest docstring to further explain how the package works,
and give a sense for why it exists.
Fixesgolang/go#36879
Change-Id: I4df4a286dc835ae56386b6d92128f7ea6c77ffb0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229782
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>