doc/usage: cmd-usage help file for askenv

help file for using askenv cmd is created.
It provides description on the command purpose,
description of arguments,
couple of examples (illustrating command usage),
configuration parameter and
possible return values.

Signed-off-by: Adarsh Babu Kalepalli <opensource.kab@gmail.com>
Add missing entry in doc/usage/index.rst.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>