mergetool: document the default for --[no-]prompt

The original motivation of using the prompt was to confirm to run a
tool on this particular (as opposed to another) path, but the user
can also take the prompt as to confirm to run this (as opposed to
some other) tool.  The latter of which of course is irritating for
those who told which exact tool to use, which is the reason why we
are flipping the default.

During the review discussion of the patch, many people (including
the maintainer) missed that a user can find the prompt useful way to
skip running the tool on particular paths.  Clarify it by adding a
brief half-sentence to the description.

Signed-off-by: Junio C Hamano <gitster@pobox.com>