ring-buffer: Update the description for ring_buffer_wait

The function changed at some point, but the description was not
updated.

Link: https://lkml.kernel.org/r/20201017095246.5170-1-hqjagain@gmail.com

Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>