Document the environment set in the invoked process.

README.rst

@@ -124,6 +124,9 @@ nothing more. Do not, for example, add spaces and command-line
 arguments; use the ``.arg()`` method for that (see below). No shell
 expansion is applied to the string.
 
+The environment of the varnishd worker process is passed to the
+program when it is invoked.
+
 The optional ``name`` parameter specifies a name to be used for the
 VDP in ``resp.filters``. If ``name`` is not set, then the VDP is
 identified by the symbol name of the object; this is the default. For

src/vdfp_pipe.vcc

@@ -108,6 +108,9 @@ nothing more. Do not, for example, add spaces and command-line
 arguments; use the ``.arg()`` method for that (see below). No shell
 expansion is applied to the string.
 
+The environment of the varnishd worker process is passed to the
+program when it is invoked.
+
 The optional ``name`` parameter specifies a name to be used for the
 VDP in ``resp.filters``. If ``name`` is not set, then the VDP is
 identified by the symbol name of the object; this is the default. For