gnatcov doc: improve the documentation for --subprograms

Specify its behavior when it refers to subprograms declared in units
that are not of interest.

Author: Jugst3r

doc/gnatcov/units_of_interest.rst

@@ -205,22 +205,27 @@ and then executing::
 
 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
 of interest.
 
-The ``--subprograms`` switch expects a ``<full_filename>:<line>`` argument,
-where ``<full_filename>`` is the full path to the subprogram source, and
-``line`` is the specification line for Ada subprograms, and the body line for
-C/C++ functions.
+The ``--subprograms`` switch expects a ``<filename>:<line>`` argument, where
+``<filename>`` is a source file name path, absolute or relative to the current
+directory, pointing to the subprogram source and ``line`` is the first line of
+the subprogram specification in Ada, and the first line of the function
+definition in C/C++.
 
 Every coverage obligation from non subprograms of interest is discarded and
 reported as *no code* in the various output formats, and the associated coverage
 data is ignored. This means that checkpoints will hold coverage data for
 subprograms of interest only.
 
+The ``--subprograms`` switch acts only on subprograms within units of interest.
+If a unit was ignored for coverage analysis through a project attribute (e.g.
+``Excluded_Units``) or through a command line switch (e.g. ``--units``), the
+designated subprogram will be ignored for coverage analysis as well.
+
 .. _gpr_context:
 
 Other switches or attributes of importance