Adds note about embedded ids into Transitions doc

I had a time figuring this out myself. `beta1` expects `comments` for
embedded ids instead of `comment_ids`. Although this
is probably going to be changed back in beta2, this change will help
people like me that will update to beta1 before beta2 is out.
Merge pull request #1151 from kurko/add_note_about_embedded_ids_into_transitions_doc

Adds note about embedded ids into Transitions doc