[webapi][XWALK-1848] Update description for webapi/3dtransforms

Impacted tests(approved): New 0, Update 6, Delete 0
Unit test platform: [Android]
Unit test result summary: Pass 6, Fail 0, Block 0
Merge pull request #329 from minxhuang/hm/xwalk-1848

[webapi][XWALK-1848] Update description for webapi/3dtransforms