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

[klausler]

Summary

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.

flang/docs/Extensions.md

* F18 allows a `USE` statement to reference a module that is defined later
   a cycle.
   This feature forestalls any risk of such a `USE` statement reading an
   obsolete module file from a previous compilation and then overwriting
   that file later.
 
 * F18 allows `OPTIONAL` dummy arguments to interoperable procedures
   unless they are `VALUE` (C865).
 
+* F18 processes the `NAMELIST` group declarations in a scope after it
+  has resolved all of the names in that scope.  This means that names
+  that appear before their local declarations do not resolve to host
+  associated objects and do not elicit errors about improper redeclarations
+  of implicitly typed entities.
+
+
 ## De Facto Standard Features
 
 * `EXTENDS_TYPE_OF()` returns `.TRUE.` if both of its arguments have the
   same type, a case that is technically implementation-defined.