Clarify the usecase for an msisdn delegate (#8734)

Signed-off-by: Adrian Wannenmacher <tfld@tfld.dev>

changelog.d/8734.doc

+Clarify the usecase for an msisdn delegate. Contributed by Adrian Wannenmacher.

docs/sample_config.yaml

@@ -1230,8 +1230,9 @@ account_validity:
 # email will be globally disabled.
 #
 # Additionally, if `msisdn` is not set, registration and password resets via msisdn
-# will be disabled regardless. This is due to Synapse currently not supporting any
-# method of sending SMS messages on its own.
+# will be disabled regardless, and users will not be able to associate an msisdn
+# identifier to their account. This is due to Synapse currently not supporting
+# any method of sending SMS messages on its own.
 #
 # To enable using an identity server for operations regarding a particular third-party
 # identifier type, set the value to the URL of that identity server as shown in the

synapse/config/registration.py

@@ -347,8 +347,9 @@ class RegistrationConfig(Config):
         # email will be globally disabled.
         #
         # Additionally, if `msisdn` is not set, registration and password resets via msisdn
-        # will be disabled regardless. This is due to Synapse currently not supporting any
-        # method of sending SMS messages on its own.
+        # will be disabled regardless, and users will not be able to associate an msisdn
+        # identifier to their account. This is due to Synapse currently not supporting
+        # any method of sending SMS messages on its own.
         #
         # To enable using an identity server for operations regarding a particular third-party
         # identifier type, set the value to the URL of that identity server as shown in the