Include/explain iterator laziness in overview documentation

[ksilin]

The documentation of Iterator#map explains the lazy nature correctly, once you know about iterator laziness already. However, the implications for side-effecting applications of map can surprise developers new to Scala (as it did surprise me)

An aside on the implications of laziness in the Iterator overview docs would be very helpful.

[ksilin]

replaces scala/bug#11145

[rkoeninger]

FYI, this is up at https://docs.scala-lang.org/overviews/collections/iterators.html#laziness