Improve accept-process-process doc

* doc/lispref/processes.texi (Accepting Output):
* src/process.c (Faccept_process_output):
Document that (accept-process-output P) can return non-nil
even after P has exited, and that it can return nil even if P
is still running (Bug#33839).

doc/lispref/processes.texi

@@ -1795,7 +1795,8 @@ until output arrives from a process.
 This function allows Emacs to read pending output from processes.  The
 output is given to their filter functions.  If @var{process} is
 non-@code{nil} then this function does not return until some output
-has been received from @var{process}.
+has been received from @var{process} or @var{process} has closed the
+connection.
 
 The arguments @var{seconds} and @var{millisec} let you specify timeout
 periods.  The former specifies a period measured in seconds and the
@@ -1820,7 +1821,9 @@ speech synthesis.
 
 The function @code{accept-process-output} returns non-@code{nil} if it
 got output from @var{process}, or from any process if @var{process} is
-@code{nil}.  It returns @code{nil} if the timeout expired before output
+@code{nil}; this can occur even after a process has exited if the
+corresponding connection contains buffered data.  The function returns
+@code{nil} if the timeout expired or the connection was closed before output
 arrived.
 @end defun
 

src/process.c

@@ -4581,8 +4581,8 @@ DEFUN ("accept-process-output", Faccept_process_output, Saccept_process_output,
        0, 4, 0,
        doc: /* Allow any pending output from subprocesses to be read by Emacs.
 It is given to their filter functions.
-Optional argument PROCESS means do not return until output has been
-received from PROCESS.
+Optional argument PROCESS means to return only after output is
+received from PROCESS or PROCESS closes the connection.
 
 Optional second argument SECONDS and third argument MILLISEC
 specify a timeout; return after that much time even if there is
@@ -4594,7 +4594,8 @@ If optional fourth argument JUST-THIS-ONE is non-nil, accept output
 from PROCESS only, suspending reading output from other processes.
 If JUST-THIS-ONE is an integer, don't run any timers either.
 Return non-nil if we received any output from PROCESS (or, if PROCESS
-is nil, from any process) before the timeout expired.  */)
+is nil, from any process) before the timeout expired or the
+corresponding connection was closed.  */)
   (Lisp_Object process, Lisp_Object seconds, Lisp_Object millisec,
    Lisp_Object just_this_one)
 {