[flang][docs] Document module name extension

[klausler]

Summary

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.)

flang/docs/Extensions.md

* Construct names like `loop` in `loop: do j=1,n` are defined to
   be "local identifiers" and should be distinct in the "inclusive
   scope" -- i.e., not scoped by `BLOCK` constructs.
   As most (but not all) compilers implement `BLOCK` scoping of construct
   names, so does f18, with a portability warning.
 * 15.6.4 paragraph 2 prohibits an implicitly typed statement function
   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
 
 * Tabs in source
 * `<>` as synonym for `.NE.` and `/=`
 * `$` and `@` as legal characters in names
 * Initialization in type declaration statements using `/values/`
 * Kind specification with `*`, e.g. `REAL*4`