[SPARK-18352][DOCS] wholeFile JSON update doc and programming guide

[felixcheung]

What changes were proposed in this pull request?

Update doc for R, programming guide. Clarify default behavior for all languages.

How was this patch tested?

manually

[SparkQA]

Test build #73708 has finished for PR 17128 at commit d542802.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #73711 has finished for PR 17128 at commit 0fd7f68.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #73729 has finished for PR 17128 at commit f5daeae.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[felixcheung]

merged to master