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>

 # Alpha specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ##

 ## help The Alpha architecture.