Document a workaround for getting past the groff infinite loop problem

Document a workaround for getting past the groff infinite loop problem
when your userland is older tahn your kernel and Makefile.inc1 therefore
thinks that you don't need to rebuild groff, when in fact you do.  This
workaround is to set OSRELDATE to 500110 before building, but only if you
have a new enough Makefile.inc1.

I've confirmed this on my laptop, which was suffering from this problem.
Since it only hits power users, we'll naively assume that such users are
smart enough to read UPDATING.