Make RP-Initiated Logout description self-contained

[Michael Jones]

RP-initiated logout can be used with any of the postMessage, Front-Channel, or Back-Channel logout methods but the current description makes it seem like it is only part of the postMessage method. The description of RP-initiated logout in the Session Management spec needs to be refactored so that it is clearly independent of the postMessage functionality.

For instance, the metadata values at https://openid.net/specs/openid-connect-session-1_0.html#OPMetadata are mixed together. The end_session_endpoint metadata should be moved into the RP-initiated logout section.

[Michael Jones]

Fixed #1085 - Split RP-Initiated Logout into its own specification

→ <<cset afa0f999e525>>

[Michael Jones]

Issue #1162 was marked as a duplicate of this issue.