fix: rename :fileset subconfig to :pattern
This name was always inaccurate, since the config accepts any pattern.
Hopefully so few people use this right now that it won't matter, but there will
now be a warning if the old config name is used.
Differential Revision: https://phab.mercurial-scm.org/D5226
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors