bug#32990: 26.1.50; isearch-forward + t-m-m/mark-active doc

[Charles A. Roelli]

> Date: Mon, 08 Oct 2018 23:07:59 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> CC: 32990@debbugs.gnu.org
> 
> Are you sure this is important to say there?  The doc string of that
> function is already way beyond annoyingly long.

It is important, but see below.

> I'd actually encourage the opposite trend: make the doc string
> shorter, and tell users to read the manual for more details.

Ok, that would be better than duplicating all the details in both the
function documentation and the manual.  I will prepare a patch.