[master] ddfda3d7c clarify that vmod return values are immutable

[Nils Goroll]

commit ddfda3d7c996e67a1894fc3955eb546ebd0b721b
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Thu Oct 25 16:03:28 2018 +0200

    clarify that vmod return values are immutable

diff --git a/doc/sphinx/reference/vmod.rst b/doc/sphinx/reference/vmod.rst
index 8978ca4f2..46ad29d5d 100644
--- a/doc/sphinx/reference/vmod.rst
+++ b/doc/sphinx/reference/vmod.rst
@@ -268,6 +268,16 @@ language representation.  Here is a description of them.
 All but the PRIV and STRING_LIST types have typedefs: VCL_INT, VCL_REAL,
 etc.
 
+Notice that all the non-native (C pointer) types are ``const``, so
+anything returned by a vmod function/method is assumed to be
+immutable. In other words, a vmod `must not` modify any data which was
+previously returned.
+
+When returning non-native values, the producing function is
+responsible for arranging memory management.  Either by freeing the
+structure later by whatever means available or by using storage
+allocated from the client or backend workspaces.
+
 ACL
 	C-type: ``const struct vrt_acl *``
 
@@ -376,12 +386,6 @@ STRING
 	If there were no "foobar" HTTP header, the vmod_foo()
 	function would be passed a NULL pointer as argument.
 
-	When used as a return value, the producing function is
-	responsible for arranging memory management.  Either by
-	freeing the string later by whatever means available or
-	by using storage allocated from the client or backend
-	workspaces.
-
 STEVEDORE
 	C-type: ``const struct stevedore *``