[flang][NFC] Document intentional non-support for an extension

F18 doesn't accept INTEGER operands to the intrinsic LOGICAL operations;
some compilers do.  This usage is not portable, and not just because it's
non-conforming -- the bit representations of LOGICAL also vary between
compilers and options.  The "MIL-STD" bit intrinsics IAND() & al. have been
avaiable since the late 70's and should be used instead.

Differential Revision: https://reviews.llvm.org/D126798