gdb/doc: extend the description for Progspace.filename

Extend the description for Progspace.filename in the documentation to
mention what the returned string is actually the filename
for (e.g. that it is the filename passed to the 'symbol-file' or
'file' command).

Also document that this attribute will be None if no symbol file is
currently loaded.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>