[flang][NFC] Document non-extension (user ELEMENTAL procedures as actual arguments)

[klausler]

Summary

Some Fortran compilers accept user-defined ELEMENTAL procedures as actual
arguments corresponding to (necessarily) non-ELEMENTAL dummy procedures;
most do not, and f18 is one of them. Document the fact that this is not
a supported extension.

[clementval]

LGTM

flang/docs/Extensions.md

* Type parameter declarations must come first in a derived type definition;
   component declarations.
 * Wrong argument types in calls to specific intrinsics that have different names than the
   related generics. Some accepted exceptions are listed above in the allowed extensions.
   PGI, Intel, and XLF support this in ways that are not numerically equivalent.
   PGI converts the arguments while Intel and XLF replace the specific by the related generic.
 * VMS listing control directives (`%LIST`, `%NOLIST`, `%EJECT`)
 * Continuation lines on `INCLUDE` lines
 * `NULL()` actual argument corresponding to an `ALLOCATABLE` dummy data object
+* User (non-intrinsic) `ELEMENTAL` procedures may not be passed as actual
+  arguments, in accordance with the standard; some Fortran compilers
+  permit such usage.
 
 ## Preprocessing behavior
 
 * The preprocessor is always run, whatever the filename extension may be.
 * We respect Fortran comments in macro actual arguments (like GNU, Intel, NAG;
   unlike PGI and XLF) on the principle that macro calls should be treated
   like function references.  Fortran's line continuation methods also work.