doc polish: remove ambiguous statement about .activate()

"Typically it suffices...", IMHO, is in some way misleading and
contradictory to ".activate mus be called in vcl_deliver at all
levels" stated before.

src/vdp_pesi.vcc

@@ -120,13 +120,11 @@ nesting), then it MUST be called on *all* levels of the response. If
 ``activate()`` is invoked at some ESI levels but not others, then the
 results are undefined, and will very likely lead to a Varnish panic.
 
-Typically it suffices to simply call ``activate()`` in
-``vcl_deliver``, since the code in ``vcl_deliver`` is executed at
-every ESI level. It is also safe, for instance, to call ``activate()``
-only if a request header is present, as in the example shown above;
-since the same request headers are set for every ESI subrequest, the
-result is the same at every ESI level. But that should *not* be done
-if you have logic that unsets the header at some ESI levels but not at
+It is also safe, for instance, to call ``activate()`` only if a
+request header is present, as in the example shown above; since the
+same request headers are set for every ESI subrequest, the result is
+the same at every ESI level. But that should *not* be done if you have
+logic that unsets the header at some ESI levels but not at
 others. Under no circumstances should the invocation of ``activate()``
 depend on the value of ``req.esi_level``, or of ``req.url`` (since
 URLs are different at different ESI levels).