regexp: clarify docs re Submatch result

regexp: clarify docs re Submatch result

Currently we say that a negative index means no match,
but we don't say how "no match" is expressed when 'Index'
is not present. Say how it is expressed.

Change-Id: I82b6c9038557ac49852ac03642afc0bc545bb4a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/175677
Reviewed-by: Ian Lance Taylor <iant@golang.org>