Updated documentation with some clarification of the optional settings

I added some additions to the docs in order to clarify that some of the optional settings only apply to the captureException call, and not the captureMessage.

Example of one more person than me that got confused by this: https://github.com/getsentry/raven-js/issues/196

If this is the expected behaviour is another discussion, but I think it is good if the docs make it clear how it currently works.