doc: Work around stylesheet bug for man build

doc: Work around stylesheet bug for man build

The upstream stylesheets for man output insert a *roff comment for an
occurrence of an indexterm, for reasons that have apparently been lost
in history.  This, however, is done incorrectly and causes some
formatting problems.  This hasn't been an issue until now, but the
reorganization of indexterm elements inside variablelists has triggered
this issue.

The upstream fix (http://sourceforge.net/p/docbook/bugs/1340/) is to
drop indexterms altogether in man output, and so we'll do the same here.