Mark the --subprograms switch as experimental

As we are still discussing some aspects of what the switch should do and
want to preserve our ability to evolve the behavior if need be.

Author: Jugst3r

doc/gnatcov/units_of_interest.rst

@@ -203,8 +203,9 @@ and then executing::
 
   gnatcov <command> --units=@units.list
 
-Conveying *subprograms* of interest
-***********************************
+Conveying *subprograms* of interest (experimental)
+**************************************************
+>>>>>>> 9544dc8f5 (Mark the --subprograms switch as experimental)
 
 |gcv| enables even finer grain control through the use of ``--subprograms``
 switch, which restricts coverage analysis to the specified list of subprograms