Add explanation API

[wnghdcjfe]

These parts are missing from the API and there is a lack of explanation in common.

• sel ~ unionBy
• unzip ~ some
• Concurrency
• Lazy(except L.filter)

[Einere]

drop method need to modify.
drop method must receive one argument(l).
so example code modified like below

dropRight(0, [1, 2, 3, 4]);
// [1, 2, 3, 4]
dropRight(1, [1, 2, 3, 4]);
// [1, 2, 3]