docs: Clarify the IdP initiated authentication documentation slightly.

- Fix the entityID in the example URL to be a valid entityID.
- Clarify the difference between `TARGET` in authentication response
  vs. `target` in authentication "request".

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2884 44740490-163a-0410-bde0-09ae8108e29a