Document why matrix_synapse_admin_well_known_client_configuration duplicates matrix_synapse_admin_configuration (roles/custom/matrix-synapse-admin/templates/config.json.j2)

roles/custom/matrix-synapse-admin/vars/main.yml

@@ -3,6 +3,10 @@
 # Configuration that can be injected into a `/.well-known/matrix/client` file,
 # to let synapse-admin users use any synapse-admin instance and still get synapse-admin auto-configured correctly.
 # See: https://github.com/etkecc/synapse-admin/pull/126
+#
+# This duplicates `roles/custom/matrix-synapse-admin/templates/config.json.j2` and is usable by other roles directly.
+# Alternatively, `matrix_synapse_admin_configuration` may have been used instead of this variable, but that one does not work reliably
+# when used outside the context of this role, because `lookup('template', 'templates/config.json.j2')` only does the right thing in the context of this role.
 matrix_synapse_admin_well_known_client_configuration:
   restrictBaseUrl: "{{ matrix_synapse_admin_config_restrictBaseUrl }}"
   asManagedUsers: "{{ matrix_synapse_admin_config_asManagedUsers }}"