[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 #3923 was:

0e26910372d349c6ff7bbaa17fc8fe0bf568c5fe

f98c53cb03f800b3d790f3866ab90f827fd131f5
Merge pull request #3937 from som-snytt/issue/8787-doc-backport

[backport] SI-8787 Backport Regex doc