Clarify updateable fields

[dreeves]

HT rperce in https://forum.beeminder.com/t/api-deadline/10666/4?u=dreev

it might be worth putting a blurb on the "update" api pages that explicitly only calls out what (if any) attributes can't be updated and says that the rest can—right now there's a list of the "special" ones like roadall, but it also includes a few "normal" parameters (like title) that make it look like ones not mentioned (like deadline) can't be updated.