[backport] SI-8787 Backport Regex doc

Backport the doc with two material changes:

1. need to use Groups to extract from Match,
so say that in lieu of the 2.11 advice that
the Regex instance can be used without
recomputing the match;

2. and relatedly, in 2.10 there is no secondary
constructor, so the doc for group names is
moved back up to the class doc.

Original doc update on PR scala#3923 was:

0e26910

f98c53c

Author: som-snytt