[Openid-specs-mobile-profile] Issue #186: Clarification for user_id and user_id_type (openid/mobile)

[naillery]

New issue 186: Clarification for user_id and user_id_type
https://bitbucket.org/openid/mobile/issues/186/clarification-for-user_id-and-user_id_type

Nicolas Aillery:

‌

```
Clarification on the presence of user_id in the User Statement Token:
-
The <spanx style="verb">user_id</spanx> is present in the User Statement Token only if the <spanx style="verb">user_id</spanx> and <spanx style="verb">user_id_type</spanx> were present in the User Questioning Request. 
+
The <spanx style="verb">user_id</spanx> MUST be present in the User Statement Token if, and only if, the <spanx style="verb">user_id</spanx> and <spanx style="verb">user_id_type</spanx> were present in the User Questioning Request. 


Clarification on the presence of user_id_type in the User Statement Token:
-
OPTIONAL. The <spanx style="verb">user_id_type</spanx> is present in the User Statement Token only if the <spanx style="verb">user_id</spanx> and <spanx style="verb">user_id_type</spanx> were present in the User Questioning Request.
+
OPTIONAL.
The <spanx style="verb">user_id_type</spanx> MUST be present in the User Statement Token if, and only if, the <spanx style="verb">user_id</spanx> and <spanx style="verb">user_id_type</spanx> were present in the User Questioning Request.
```

‌