Add matrix_admin example

roles/matrix-base/defaults/main.yml

@@ -9,6 +9,7 @@
 matrix_domain: ~
 
 # The optional matrix admin MXID, used in bridges' configs to set bridge admin user
+# Example value: "@someone:{{ matrix_admin }}"
 matrix_admin: ''
 
 # This will contain the homeserver implementation that is in use.