Improve zh-doc, 'sql' section of Mapper.
Improve zh-doc, first part of 'Parameters' section of Mapper.
Improve zh-doc, last part of 'Paramters' sectiion and 'String substitution' section of Mapper.
Improve zh-doc, 'String Substitution' of Mapper.
Merge pull request #254 from wangdx/master

Improve zh-doc for Mapper.