[master] 1b7d8f457 doc: Mention the VCL_STRANDS red tape

[Dridi Boukelmoune]

commit 1b7d8f457b8e7546ebd825268efe39db0ef82efb
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Wed Jan 13 08:50:09 2021 +0100

    doc: Mention the VCL_STRANDS red tape

diff --git a/doc/sphinx/reference/vmod.rst b/doc/sphinx/reference/vmod.rst
index 19f2ac79e..ee9d4f991 100644
--- a/doc/sphinx/reference/vmod.rst
+++ b/doc/sphinx/reference/vmod.rst
@@ -442,6 +442,9 @@ STRANDS
 	* ``int n``: the number of strings
 	* ``const char **p``: the array of strings with `n` elements
 
+	A VMOD should never hold onto strands beyond a function or method
+	execution. See ``include/vrt.h`` for the details.
+
 TIME
 	C-type: ``double``