[flang] document a non-extension

Original-commit: flang-compiler/f18@625daf5218
Reviewed-on: https://github.com/flang-compiler/f18/pull/590
Tree-same-pre-rewrite: false

flang/documentation/Extensions.md

@@ -135,6 +135,10 @@ Extensions and legacy features deliberately not supported
 * Mixing INTEGER types as operands to bit intrinsics (e.g., IAND); only two
   compilers support it, and they disagree on sign extension.
 * Module & program names that conflict with an object inside the unit (PGI only).
+* When the same name is brought into scope via USE association from
+  multiple modules, the name must refer to a generic interface; PGI
+  allows a name to be a procedure from one module and a generic interface
+  from another.
 
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