[flang][docs] Document module name extension

Add an entry to the Extensions document describing how we
don't care about conflicts between module names and non-global
items.  (This is a case where it would be a nontrivial amount
of work to catch an "error" that is only a standard conformance
issue, not anything that would prevent a program from working.)

Differential Revision: https://reviews.llvm.org/D147387

flang/docs/Extensions.md

@@ -88,6 +88,9 @@ end
   from sharing the same name as a symbol in its scope's host, if it
   has one.
   We accept this usage with a portability warning.
+* A module name from a `USE` statement can also be used as a
+  non-global name in the same scope.  This is not conforming,
+  but it is useful and unambiguous.
 
 ## Extensions, deletions, and legacy features supported by default