[SPARK-4930][SQL][DOCS]Update SQL programming guide, CACHE TABLE is eager

[luogankun]

CACHE TABLE tbl is now eager by default not lazy

[AmplabJenkins]

Can one of the admins verify this patch?

[liancheng]

Thanks for fixing this! Would you mind to add a paragraph about the newly introduced CACHE LAZY TABLE [AS SELECT] ... syntax which preserves laziness?

[luogankun]

ok

[marmbrus]

Thanks! Merged to master.