[Minor] Fix scala doc

Minor fix for an obvious scala doc error.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #3751 from viirya/fix_scaladoc and squashes the following commits:

03fddaa [Liang-Chi Hsieh] Fix scala doc.
[SPARK-4920][UI]:current spark version in UI is not striking.

It is not convenient to see the Spark version. We can keep the same style with Spark website.

![spark_version](https://cloud.githubusercontent.com/assets/7402327/5527025/1c8c721c-8a35-11e4-8d6a-2734f3c6bdf8.jpg)

Author: genmao.ygm <genmao.ygm@alibaba-inc.com>

Closes #3763 from uncleGen/master-clean-141222 and squashes the following commits:

0dcb9a9 [genmao.ygm] [SPARK-4920][UI]:current spark version in UI is not striking.
[SPARK-4818][Core] Add 'iterator' to reduce memory consumed by join

In Scala, `map` and `flatMap` of `Iterable` will copy the contents of `Iterable` to a new `Seq`. Such as,
```Scala
  val iterable = Seq(1, 2, 3).map(v => {
    println(v)
    v
  })
  println("Iterable map done")

  val iterator = Seq(1, 2, 3).iterator.map(v => {
    println(v)
    v
  })
  println("Iterator map done")
```
outputed
```
1
2
3
Iterable map done
Iterator map done
```
So we should use 'iterator' to reduce memory consumed by join.

Found by Johannes Simon in http://mail-archives.apache.org/mod_mbox/spark-user/201412.mbox/%3C5BE70814-9D03-4F61-AE2C-0D63F2DE4446%40mail.de%3E

Author: zsxwing <zsxwing@gmail.com>

Closes #3671 from zsxwing/SPARK-4824 and squashes the following commits:

48ee7b9 [zsxwing] Remove the explicit types
95d59d6 [zsxwing] Add 'iterator' to reduce memory consumed by join