[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

 * 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