gdb/python: improve the auto help text for gdb.Parameter

This commit attempts to improve the help text that is generated for
gdb.Parameter objects when the user fails to provide their own
documentation.

Documentation for a gdb.Parameter is currently pulled from two
sources: the class documentation string, and the set_doc/show_doc
class attributes.  Thus, a fully documented parameter might look like
this:

  class Param_All (gdb.Parameter):
     """This is the class documentation string."""

     show_doc = "Show the state of this parameter"
     set_doc = "Set the state of this parameter"

     def get_set_string (self):
        val = "on"
        if (self.value == False):
           val = "off"
        return "Test Parameter has been set to " + val

     def __init__ (self, name):
        super (Param_All, self).__init__ (name, gdb.COMMAND_DATA, gdb.PARAM_BOOLEAN)
        self._value = True

  Param_All ('param-all')

Then in GDB we see this:

  (gdb) help set param-all
  Set the state of this parameter
  This is the class documentation string.

Which is fine.  But, if the user skips both of the documentation parts
like this:

  class Param_None (gdb.Parameter):

     def get_set_string (self):
        val = "on"
        if (self.value == False):
           val = "off"
        return "Test Parameter has been set to " + val

     def __init__ (self, name):
        super (Param_None, self).__init__ (name, gdb.COMMAND_DATA, gdb.PARAM_BOOLEAN)
        self._value = True

  Param_None ('param-none')

Now in GDB we see this:

  (gdb) help set param-none
  This command is not documented.
  This command is not documented.

That's not great, the duplicated text looks a bit weird.  If we drop
different parts we get different results.  Here's what we get if the
user drops the set_doc and show_doc attributes:

  (gdb) help set param-doc
  This command is not documented.
  This is the class documentation string.

That kind of sucks, we say it's undocumented, then proceed to print
the documentation.  Finally, if we drop the class documentation but
keep the set_doc and show_doc:

  (gdb) help set param-set-show
  Set the state of this parameter
  This command is not documented.

That seems OK.

So, I think there's room for improvement.

With this patch, for the four cases above we now see this:

  # All values provided by the user, no change in this case:
  (gdb) help set param-all
  Set the state of this parameter
  This is the class documentation string.

  # Nothing provided by the user, the first string is now different:
  (gdb) help set param-none
  Set the current value of 'param-none'.
  This command is not documented.

  # Only the class documentation is provided, the first string is
  # changed as in the previous case:
  (gdb) help set param-doc
  Set the current value of 'param-doc'.
  This is the class documentation string.

  # Only the set_doc and show_doc are provided, this case is unchanged
  # from before the patch:
  (gdb) help set param-set-show
  Set the state of this parameter
  This command is not documented.

The one place where this change might be considered a negative is when
dealing with prefix commands.  If we create a prefix command but don't
supply the set_doc / show_doc strings, then this is what we saw before
my patch:

  (gdb) python Param_None ('print param-none')
  (gdb) help set print
  set print, set pr, set p
  Generic command for setting how things print.

  List of set print subcommands:

  ... snip ...
  set print param-none -- This command is not documented.
  ... snip ...

And after my patch:

  (gdb) python Param_None ('print param-none')
  (gdb) help set print
  set print, set pr, set p
  Generic command for setting how things print.

  List of set print subcommands:

  ... snip ...
  set print param-none -- Set the current value of 'print param-none'.
  ... snip ...

This seems slightly less helpful than before, but I don't think its
terrible.

Additionally, I've changed what we print when the get_show_string
method is not provided in Python.

Back when gdb.Parameter was first added to GDB, we didn't provide a
show function when registering the internal command object within
GDB.  As a result, GDB would make use of its "magic" mangling of the
show_doc string to create a sentence that would display the current
value (see deprecated_show_value_hack in cli/cli-setshow.c).

However, when we added support for the get_show_string method to
gdb.Parameter, there was an attempt to maintain backward compatibility
by displaying the show_doc string with the current value appended, see
get_show_value in py-param.c.  Unfortunately, this isn't anywhere
close to what deprecated_show_value_hack does, and the results are
pretty poor, for example, this is GDB before my patch:

  (gdb) show param-none
  This command is not documented. off

I think we can all agree that this is pretty bad.

After my patch, we how show this:

  (gdb) show param-none
  The current value of 'param-none' is "off".

Which at least is a real sentence, even if it's not very informative.

This patch does change the way that the Python API behaves slightly,
but only in the cases when the user has missed providing GDB with some
information.  In most cases I think the new behaviour is a lot better,
there's the one case (noted above) which is a bit iffy, but I think is
still OK.

I've updated the existing gdb.python/py-parameter.exp test to cover
the modified behaviour.

Finally, I've updated the documentation to (I hope) make it clearer
how the various bits of help text come together.

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