Document that 'make-process' mixes the output streams

* doc/lispref/processes.texi (Asynchronous Processes):
* src/process.c (Fmake_process): Document that standard error is mixed
with standard output if STDERR is nil.

* test/src/process-tests.el (make-process/mix-stderr): New unit test.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/process.c b/src/process.c
index ed2cab7b51f..c357a8bdc33 100644
--- a/src/process.c
+++ b/src/process.c
@@ -1657,7 +1657,8 @@ to use a pty, or nil to use the default specified through
 
 :stderr STDERR -- STDERR is either a buffer or a pipe process attached
 to the standard error of subprocess.  Specifying this implies
-`:connection-type' is set to `pipe'.
+`:connection-type' is set to `pipe'.  If STDERR is nil, standard error
+is mixed with standard output and sent to BUFFER or FILTER.
 
 usage: (make-process &rest ARGS)  */)
   (ptrdiff_t nargs, Lisp_Object *args)