[Bug 1087697] man page inaccuracy about --sig-proxy and --tty

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1087697

--- Comment #8 from Matthew Heon  ---
Considering there's no action on the upstream bug, I've started work on a fix.

Right now, it's working for all signals save 17 and 20-22. 17 is expected,
Docker itself blocks that, but 20-22 not going through is a regression. I'm
debugging this, and once that issue is fixed, I'll submit the patch upstream.