Add note about how optional messages work to documentation

[morinted]

I was glad to see that expect works with custom messages, but had to look at the code to see exactly how and when they are fired, and how I should format them in order to stay consistent with the library's style.

It might be good to add a note explaining the tense of the default messages ("expected x") and how the optional message replaces this one, and how this might be useful (e.g. giving more meaning than "expected false to be truthy")