doc: clarify the intent of the renormalize option in the merge machinery

doc: clarify the intent of the renormalize option in the merge machinery

The -X renormalize (or merge.renormalize config) option is intended to
reduce conflicts due to normalization of newer versions of history.  It
does so by renormalizing files that it is about to do a three-way
content merge on.  Some folks thought it would renormalize all files
throughout the tree, and the previous wording wasn't clear enough to
dispell that misconception.  Update the docs to make it clear that the
merge machinery will only apply renormalization to files which need a
three-way content merge.

(Technically, the merge machinery also does renormalization on
modify/delete conflicts, in order to see if the modification was merely
a normalization; if so, it can accept the delete and not report a
conflict.  But it's not clear that this piece needs to be explained to
users, and trying to distinguish it might feel like splitting hairs and
overcomplicating the explanation, so we leave it out.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>