[flang] Update Extensions.md

Add note about type parameter declarations needing to come first in a derived type definition.

Original-commit: flang-compiler/f18@01358ac86e
Reviewed-on: https://github.com/flang-compiler/f18/pull/619

flang/documentation/Extensions.md

@@ -139,6 +139,9 @@ Extensions and legacy features deliberately not supported
   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.
+* Type parameter declarations must come first in a derived type definition;
+  some compilers allow them to follow `PRIVATE`, or be intermixed with the
+  component declarations.
 
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