Remove the docs for now

As previously, cleanly remove the docs option until Doxygen can
be used correctly.
Remove the manpages option

The manpages option doesn't matter as by default manpages are getting
installed, the "doxygen-man" option is another thing and requires
getting doxygen working as expected.

Maybe someone still may want it to install without manpages, then I
might revisit this option.
Simplify descriptions

Describte the function, not on what it depends
Previous text was obtained from lldpd's configure.
Tune the proprietary protocol support description.