update: do not use the term 'update' when mentioning reverting one file
and give a more precise hint for how to revert such a file
I'm using the term 'revision' instead of 'changeset' in this change to be
consistent with the REV we use in the synopsis.
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case