The roff ascii.gz documentation installed to /usr/share/doc

The roff ascii.gz documentation installed to /usr/share/doc
was removed in r318881 when roff was removed from the base
system.

This results in the doc.txz distribution set containing a
single directory (./) which is empty.

Remove the "Additional documentation" option from the menu
selection of bsdinstall(8), as the plain-text documentation
installed in /usr/share/doc is installed as part of the
packageworld target.

The doc entry has not been removed from EXTRA_DISTRIBUTIONS
in Makefile.inc1, in case its removal triggers an issue with
freebsd-update(8), which is currently aware of the world/doc
component, so the empty doc.txz continues to be created as
a precaution.

Noticed by:	rgrimes
MFC after:	2 days
MFC before:	12.0-RC1
Sponsored by:	The FreeBSD Foundation