[flang][docs] Document non-supported VMS extensions

[klausler]

Summary

The prescanner does not support VMS Fortran listing control directives
(%LIST, %NOLIST, %EJECT) or continuation lines on INCLUDE lines.

flang/docs/Extensions.md

* When the same name is brought into scope via USE association from
   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.
 * 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
 
 ## 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.