Update documentation and tutorial prose to reflect recent changes

[joshuathayer]

We're about to merge (to master) many changes to Metaprob which render documentation and example prose out-of-date.

Update all documentation, tutorial prose, and other non-code artifacts to reflect the changes in Metaprob.

[jar600]

May I please reinforce this issue and urge priority for it. I wanted to know the return value of at, so I looked at the README to find the documentation. The language reference has no mention of at. So now I will have to read the source code or something.