Docs: Document URI policy exception for roles

Relates to special URI template for roles as implemented for gh-31.

docs/antora/modules/ROOT/pages/methodology.adoc

@@ -134,6 +134,14 @@ instead of ID in its trailer, as it may not have a short ID that is
 sensible to use/read (however we may not have enforced this rule
 strongly)
 
+There are exceptions to this policy, namely in the _trailer_ segment, as that
+is what lends uniqueness to a resource, and determines whether instances being
+created from subject URI templates in the technical RML rules are correct. One
+such exception are epo:AgentInRole instances, which
+https://github.com/OP-TED/ted-rdf-mapping-eforms/issues/31[require a carefully
+constructed URI] seeded with information about the related party (a
+foaf:Agent).
+
 **Note:** Wherever _URI_ is mentioned,
 https://www.w3.org/2001/Talks/0912-IUC-IRI/paper.html#:~:text=In%20principle%2C%20the%20definition%20of,us%2Dascii%20characters%20in%20URIs[IRI]
 is meant. Also, the generation of hashes is done _online_ against a