[flang][NFC] Document semantics of an ambiguous/non-portable use case

We intentionally process NAMELIST groups in a scope after having
resolved all of the names in that scope.  This means that a name
whose first appearance in a scope is in the NAMELIST group resolves
to a local object, if any, rather than to any host associated object.
The standard is unclear on this point, and there is no clear
precedent in other compilers.

This patch doesn't implement this choice -- that was done long ago --
but just documents the behavior in Extensions.md.

Differential Revision: https://reviews.llvm.org/D154375