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

This compiler (like all others) allows OPTIONAL dummy arguments to
appear as arguments to some intrinsic functions that are specified
as disallowing them; document this interpretation better.

Author: klausler

flang/docs/Extensions.md

@@ -189,6 +189,20 @@ end
   `PROCEDURE(), POINTER, NOPASS` derived type components.
   Such procedures may *not* be referenced as implicitly typed functions
   without first being associated with a function pointer.
+* Some intrinsic functions (`COUNT`, `LBOUND`, `LCOBOUND`, `TRANSFER`,
+  `UBOUND`, and `UCOBOUND`) have arguments (usually `DIM=`) that are documented
+  as not allowing `OPTIONAL` dummy arguments to appear as their values.
+  This prohibition appeared on the `DIM=` arguments of more
+  intrinsic functions in earlier revisions of the ISO standard.
+  (Perhaps these are meant to avoid misunderstanding these arguments,
+  which appear in square brackets in the synopses, as if their dynamic
+  presence at runtime could affect the semantics of the intrinsic in
+  the same way as the static presence or absence of the argument does
+  at compilation time, which would not be possible.)
+  No compiler seems to enforce this requirement, and we interpret it
+  to mean that a`OPTIONAL` dummy argument may appear but must be present
+  during execution, just as a pointer argument must be associated or an
+  allocatable argument must be allocated.
 
 ## Extensions, deletions, and legacy features supported by default