stty: document the 'discard' character setting

stty: document the 'discard' character setting

The equivalent of this is 'flush', but that was never documented
as an option (though was output with stty -a).  Therefore use
the more descriptive name, also generally used on BSD systems.
Note even though this setting seems ineffective on Linux, supporting
the setting is useful to allow terminal programs to receive
the default ^O character code.

* doc/coreutils.texi (stty invocation): Document the 'discard' option.
* src/stty.c (struct control_info): Add 'discard'; same as 'flush'.
(display_all): Show 'discard' rather than 'flush' char.
(display_changed): Likewise.
(usage): Document the 'discard' option.