[5.2] Builder: Remove code duplication, Add code comments

[vlakoff]

• Remove duplicate code of chunk(). Added in 11d3e98. Spotted by n7olkachev in [5.2] Add when method to EloquentBuilder #13726 (comment). I can't think of any purpose for this duplication.
• Add code comments about when() callback (sorry no "-3" comments, I don't get how you manage to write them). See [5.2] Optional query builder calls via a when() method #12878 for the query builder, then [5.2] Add when method to EloquentBuilder #13726 for the eloquent builder. There is also the late [5.2] Make $query->when() return instance if callback return null #13091 😢

[vlakoff]

Mmm, got it. Eloquent builder has to call its get method, not the query builder's one, so it gets a Collection.