GDB: Add references to erased args in cli-decode.c

Complement commit 1d7fe7f0 ("gdb: Introduce setting construct within
cmd_list_element") and commit 70299171 ("gdb: Have setter and getter
callbacks for settings") and update inline documentation accordingly for
`add_set_or_show_cmd' and `add_setshow_cmd_full_erased', documenting the
`args' parameter and removing references to `var', `set_setting_func'
and `get_setting_func'.

Approved-By: Simon Marchi <simon.marchi@efficios.com>