[Openid-specs-ab] Issue #2127: [Federation] Editorial: Improve 5.1. Entity Type Identifiers intro (openid/connect)

[Vladimir Dzhuvinov]

New issue 2127: [Federation] Editorial: Improve 5.1. Entity Type Identifiers intro
https://bitbucket.org/openid/connect/issues/2127/federation-editorial-improve-51-entity

Vladimir Dzhuvinov:

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.
```

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