poll: Clarify when FALSE is returned from read/write_control()

poll: Clarify when FALSE is returned from read/write_control()

And also mention what the expected values of errno are going to be.

write_control() will only ever return FALSE if there was a critical error. It
will never return because of EINTR, EAGAIN or EWOULDBLOCK.

read_control() will return FALSE if there was no byte to read, in which case
errno would be EWOULDBLOCK.
In all other cases there was a critical error.

https://bugzilla.gnome.org/show_bug.cgi?id=750397