[SPARK-7704] Updating Programming Guides per SPARK-4397

The change per SPARK-4397 makes implicit objects in SparkContext to be found by the compiler automatically. So that we don't need to import the o.a.s.SparkContext._ explicitly any more and can remove some statements around the "implicit conversions" from the latest Programming Guides (1.3.0 and higher)

Author: Dice <poleon.kd@gmail.com>

Closes #6234 from daisukebe/patch-1 and squashes the following commits:

b77ecd9 [Dice] fix a typo
45dfcd3 [Dice] rewording per Sean's advice
a094bcf [Dice] Adding a note for users on any previous releases
a29be5f [Dice] Updating Programming Guides per SPARK-4397