[flang][NFC] Document an intentional violation of the standard

The Fortran standard committees passed an "interp" request at their
June 2024 meetings that is contrary to what every other Fortran
compiler that I tried (6) does with an ambiguous case (parent component
naming when the base type has been renamed via USE association).
Document this case in flang/docs/Extensions.md as an intentional
instance of non-conformance chosen for portability and better
usability.

Author: klausler

flang/docs/Extensions.md

@@ -134,6 +134,13 @@ end
   implicitly simply appearing in an asynchronous data transfer statement,
   without the attribute being visible in the procedure's explicit
   interface.
+* When the name of an extended derived type's base type is the
+  result of `USE` association with renaming, the name of the extended
+  derived type's parent component is the new name by which the base
+  is known in the scope of the extended derived type, not the original.
+  This interpretation has usability advantages and is what six other
+  Fortran compilers do, but is not conforming now that J3 approved an
+  "interp" in June 2024 to the contrary.
 
 ## Extensions, deletions, and legacy features supported by default