cmd/compile: minor doc enhancements

Add a little more detail to the ssa README relating to GOSSAFUNC.

Update the -d=ssa help section to give a little more detail on what
to expect with applying the /debug=X qualifier to a phase.

Change-Id: I7027735f1f2955dbb5b9be36d9a648e8dc655048
Reviewed-on: https://go-review.googlesource.com/c/go/+/315229


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>