Say (accept-process-output P)'s result pertains to P if P is non-nil.

Say (accept-process-output P)'s result pertains to P if P is non-nil.

* doc/lispref/processes.texi (Accepting Output):
* src/process.c (Faccept_process_output)
(wait_reading_process_output): Mention that if PROCESS is non-nil,
the return value is about PROCESS, not about other processes.