Document request parameter handling (#640)

* document request paramter handling #636

* document request paramter handling #636

* apply the improvements proposed by @jirmac00

* - wording and grammar changes

* - merging the documentation into the ZDG

* - point to the Zope Developer's Guide

* - updates following Dieter's suggestions

* remove `src/ZPublisher/request_params.rst` as its content is now integrated in `docs/zdgbook/ObjectPublishing.rst`

* bring request lookup order documentation in line with reality

* remove items from the "converter" list which are now classified as "aggregator"
use the same formatting type for "aggregator"s as for "converter"s

* Follow suggestion from icemac

Co-Authored-By: Michael Howitz <mh@gocept.com>

* Follow suggestion from icemac

Co-Authored-By: Michael Howitz <mh@gocept.com>

* Follow suggestion from icemac

Co-Authored-By: Michael Howitz <mh@gocept.com>

CHANGES.rst

@@ -22,6 +22,10 @@ Features
   registration of a ``ZPublisher.interfaces.IXmlrpcChecker`` utility
   (`#620 <https://github.com/zopefoundation/Zope/issues/620>`_).
 
+- Document request parameter handling
+  (`#636 <https://github.com/zopefoundation/Zope/issues/636>`_).
+
+
 Fixes
 +++++