Migrate example definitions from `specify` to `it`.

Under protest, but the `it` syntax seems to be clearly favored. Both
`context` and `describe` are retained, with `context` reserved for
changes to the environment rather than the direct subject of the test
(e.g. adding sibling records to a collection).

Current spec status:
1.9.3: 196 examples, 0 failures in 0.07358 seconds.
2.0.0: 211 examples, 0 failures in 0.08172 seconds.
2.1.0: 234 examples, 0 failures in 0.07603 seconds.
Update documentation.