Improve documentation for querying collections

[odrotbohm]

Trying to use the collection query feature of Querydsl I've discovered a few places in the reference documentation that would benefit from an update to improve the getting started experience.

1. Mentioning that querydsl-collections library would be helpful.
2. It's not obvious where the from(…) call in the examples is actually coming from. The documentations mentions a static import for Alias but that is not the type needed. Once you've figured out 1. you might find MiniApi to actually contain the said method.

[timowest]

Released in 2.7.0