Updated API reference doc for Single-Source RS

Bug: 29544612
Bug: 29879448

This is auto-generated by running frameworks/rs/api/generate.sh, but
only for rsForEach() and rsCreateAllocation().
Editings to the .spec files in frameworks/rs/api were made based on
feedbacks here.

Staging server:
http://yangni.mtv.corp.google.com/guide/topics/renderscript/reference/rs_for_each.html
http://yangni.mtv.corp.google.com/guide/topics/renderscript/reference/rs_allocation_create.html

Change-Id: I072f2f3fffaa9a1a2af101fa8061f61089daf208