Minor mdoc fixes.

Approved by:	re (blanket)
Properly remove c++filt.

The c++filt binary is only installed if ${MK_GCC} == yes && ${MK_CXX} ==
yes. This means that it should be removed if ${MK_GCC} == no ||
${MK_CXX} == no. In its current form, it actually uses a conjunction
instead of a disjunction.

As there is already a separate block for ${MK_CXX} == no listing
c++filt, simply remove the conditional entirely.

Approved by:	re (gjb)