help: explain in `hg help flags` that unambiguous prefixes are allowed
I used `hg commit --amend` as an example because that's the most
frequently used flag I could think of that doesn't yet have a short
form.
Differential Revision: https://phab.mercurial-scm.org/D8607
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors