callbacks: Improve the documentation of ssh_threads_get_noop().
callbacks: Improve the documentation of ssh_threads_get_noop().

BUG: https://red.libssh.org/issues/123

mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh