merge: improve documentation of fbid dict used for merge bid
I improved the comments explaning what the dict contains meanwhile organizing
the comment structure which prevents some confusion.
Due to formatting issues, the empty dict was wrapped in `()` which might decieve
in thinking that it's a tuple of dict until you decide to find a comma.
Differential Revision: https://phab.mercurial-scm.org/D8818
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors