Added more documentation about the executable argument.
Fixes #1056441.
diff --git a/Doc/lib/libsubprocess.tex b/Doc/lib/libsubprocess.tex
index 14e68aa..01f64ae 100644
--- a/Doc/lib/libsubprocess.tex
+++ b/Doc/lib/libsubprocess.tex
@@ -68,6 +68,13 @@
use the system default, which usually means fully buffered. The default
value for \var{bufsize} is \constant{0} (unbuffered).
+The \var{executable} argument specifies the program to execute. It is
+very seldom needed: Usually, the program to execute is defined by the
+\var{args} argument. If \var{shell=True}, the \var{executable}
+argument specifies which shell to use. On \UNIX{}, the default shell
+is /bin/sh. On Windows, the default shell is specified by the COMSPEC
+environment variable.
+
\var{stdin}, \var{stdout} and \var{stderr} specify the executed
programs' standard input, standard output and standard error file
handles, respectively. Valid values are \code{PIPE}, an existing file