The mandoc(1) manual already mentions that -T man output mode

The mandoc(1) manual already mentions that -T man output mode
neither supports tbl(7) nor eqn(7) input.
If an input file contains such code anyway, tell the user
rather than failing an assert(3)ion.

Fixing a crash reported by Bjarni Ingi Gislason <bjarniig at rhi dot hi dot is>
in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901636 which the
Debian maintainer of mandoc, Michael at Stapelberg dot ch, forwarded to me.