Rewise warnings and notes in narrative docs

[bsipocz]

This is a reminder issue to go through the narrative docs and revisit the big red warnings, e.g. http://docs.astropy.org/en/latest/timeseries/ before the release.

cc @astrofrog

[bsipocz]

The uncertainty warning has been revised before 4.0, but we agreed that the timeseries etc can be done in a bugfix release, too, so I'm removing the milestone and label.