docs: clarify what git-rebase's "-p" / "--preserve-merges" does

docs: clarify what git-rebase's "-p" / "--preserve-merges" does

Ignoring a merge can be read as ignoring the changes a merge commit
introduces altogether, as if the entire side branch the merge commit
merged was removed from the history.  But that is not what happens
if "-p" is not specified.  What happens is that the individual
commits a merge commit introduces are replayed in order, and only
any possible merge conflict resolutions or manual amendments to the
merge commit are ignored.

Get this straight in the docs.

Also, do not say that merge commits are *tried* to be recreated. As that is
true almost everywhere it is better left unsaid.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>