[SPARK-6957] [SPARK-6958] [SQL] improve API compatibility to pandas

[davies]

select(['cola', 'colb'])

groupby(['colA', 'colB'])
groupby([df.colA, df.colB])

df.sort('A', ascending=True)
df.sort(['A', 'B'], ascending=True)
df.sort(['A', 'B'], ascending=[1, 0])

cc @rxin

[SparkQA]

Test build #30440 has finished for PR 5544 at commit bcbbcab.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[SparkQA]

Test build #30443 has finished for PR 5544 at commit adb2816.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[rxin]

can we add some docstring here to explain what this does?

[rxin]

Looks good to me, except some comments on documentation. Thanks for doing this!

[SparkQA]

Test build #30456 has finished for PR 5544 at commit 4944058.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[rxin]

Thanks. I'm going to merge this and fix some minor doc issues.