docs/known-issues: add the infamous "mixed implicit and normal rules"

make-3.82 now refuses to have both normal and implicit rules in the
same definition, so document the issue with a known workaround.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 python

 import sys

 sys.path.insert(0, '@@PREFIX@@/share/gcc-@@VERSION@@/python')

 from libstdcxx.v6.printers import register_libstdcxx_printers

 register_libstdcxx_printers (None)

 end