Docs: Add a note on custom validation contexts.

During one of DHH's coding ping pongs, I noticed he used custom validation contexts, which I'd never seen before. I searched for more information in the Rails docs, but couldn't find any -- they only referred to `:create` and `:update`. 

`AM::Validations.validates` actually had the right documentation, so I copied (and tweaked) that into the methods with out-of-date documentation, to make it more consistent and easy to find!