Update LTI settings information

@mhoeber @lamagnifica Please review.

@Lyla-Fischer Does anything other than the UI information need to be updated as a result of [BLD-855](https://edx-wiki.atlassian.net/browse/BLD-855)? I.e., is the rest of the [LTI topic](http://ca.readthedocs.org/en/latest/exercises_tools/lti_component.html) still accurate?