netgdb(4): update list of required kernel options

The man page claims that netgdb will be enabled automatically with the
presence of the DDB, GDB, and INET options. Based on the logic in
conf/files, this is not the case. Update the manpage to list all
of the options required to include netgdb.

Reviewed by: pauamma, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D37330

(cherry picked from commit d5f3e80f5a0b64ccde9ab6856766fc565e795393)