ArgparsedCommand: fix `help cmd` and `cmd --help` behavior (#1108)
ArgparsedCommand: fix `help cmd` and `cmd --help` behavior (#1108)

* ArgparsedCommand: fix `help cmd` and `cmd --help` behavior

Before this commit there was always a mismatch of what was displayed
when the user did `<command> --help` or `help <command>`.

With those changes, we fetch the help string from the argument parser
and render it as the command object's `self.__doc__`, so that it will be
displayed during `help <command>`.

Previously, we only displayed the command description during help.

* fix the pwndbg [filter] command that was broken in previous commit
pwndbg/pwndbg,pwndbg/pwndbg,pwndbg/pwndbg,pwndbg/pwndbg