Docs on sig-proxy are wrong, we support TTY

Also, we don't proxy SIGURG (Golang uses it internally for waking
threads, so Go processes get it constantly (see [1] for more
details).

[1] golang/go#37942

Signed-off-by: Matt Heon <[email protected]>

Author: mheon

docs/source/markdown/options/sig-proxy.md

@@ -4,4 +4,4 @@
 ####> are applicable to all of those.
 #### **--sig-proxy**
 
-Proxy received signals to the container process (non-TTY mode only). SIGCHLD, SIGSTOP, and SIGKILL are not proxied.
+Proxy received signals to the container process. SIGCHLD, SIGURG, SIGSTOP, and SIGKILL are not proxied.