* Clarify data stream recommendations and best practices

Our documentation around data streams versus aliases could be interpreted in a way where someone doing *any* updates thinks they need to use an alias with indices instead of a data stream. This commit enhances the documentation around these areas to determine the correct abstraction in a more concrete way. It also tries to clarify that data streams still allow updates to the backing indices, and that a difference is last-write-wins versus first-write-wins.