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, 3 insertions, 1 deletions

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index af177e053cc..3e26f577982 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -681,7 +681,9 @@ a default sentinel will be used, which can be overridden later.
 @item :stderr @var{stderr}
 Associate @var{stderr} with the standard error of the process.  A
 non-@code{nil} value should be either a buffer or a pipe process
-created with @code{make-pipe-process}, described below.
+created with @code{make-pipe-process}, described below.  If
+@var{stderr} is @code{nil}, standard error is mixed with standard
+output, and both are sent to @var{buffer} or @var{filter}.
 @end table
 
 The original argument list, modified with the actual connection