help: document meaning of '%' in graphlog output
I added the feature in
14d0e89520a2 (graphlog: use '%' for other
context in merge conflict, 2020-01-28), but I didn't think to look for
documentation until today (because an internal user asked about it).
Differential Revision: https://phab.mercurial-scm.org/D8649
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors