[Federation] Editorial: Improve 5.1. Entity Type Identifiers intro

Proposed intro that first describes the nature of the Entity Type Identifier and then the particular identifiers defined in the section:

The Entity Type Identifier uniquely identifies the Entity Type
of a federation participant and the metadata format for that 
Entity Type. This section defines a 
<spanx style="verb">federation_entity</spanx> Entity Type 
Identifier as well as identifiers for OpenID Connect and OAuth 
2.0 Federation Entities.

‌

Comments (2)