[ADD] doc: guide to the new API

Raw API documentation is not sufficient for people to learn the working
principles of the API, especially when they already know the old one. And some
things have changed since the opendays so the presentations don't reflect the
current state of affairs.

Closes #3483

doc/glossary.rst

@@ -13,6 +13,9 @@
         ``account.invoice_graph``). From within a module, the
         :samp:`{module}.` prefix can be left out.
 
+        Sometimes referred to as "xml id" or ``xml_id`` as XML-based
+        :ref:`reference/data` make extensive use of them.
+
     format string
         inspired by `jinja variables`_, format strings allow more easily
         mixing literal content and computed content (expressions): content