Document E164 lack of support for phone extensions

While changing the default is likely to cause large breakage, we can at
least advertise that limitation and allow people to pick the
INTERNATIONAL format (E163) if they need to support extensions.

docs/reference.rst

@@ -366,9 +366,9 @@ Default: ``"E164"``.
 
 Choices:
 
-- ``"E164"``,
-- ``"INTERNATIONAL"``,
-- ``"RFC3966"``,
+- ``"E164"`` (public international numbers),
+- ``"INTERNATIONAL"`` (international numbers, possibly with phone extensions),
+- ``"RFC3966"`` (international numbers, possibly with phone extensions),
 - ``"NATIONAL"`` (discouraged, requires :setting:`PHONENUMBER_DEFAULT_REGION`).
 
 .. setting:: PHONENUMBER_DEFAULT_REGION
@@ -392,7 +392,7 @@ Default: ``"E164"``.
 
 Choices:
 
-- ``"E164"``,
+- ``"E164"`` (no support of phone extensions),
 - ``"INTERNATIONAL"``,
 - ``"RFC3966"``,
 - ``"NATIONAL"`` (discouraged, fails to represent international phone numbers).