[flang][docs] Document module name extension

[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