added docs description of abstract db types in migration

replaces #618
Merge branch 'master' of github.com:yiisoft/yii2

* 'master' of github.com:yiisoft/yii2:
  minor doc fix.
  minor doc improvement.
  Add echo into docs for link pager
  Update migration.md. Fix missed quotes
  handle array in importNamespaces.
  import namespaces.
  import namespaces.
  renamed autoload to classmap.
  Added autoload command.
  Rewording
  Added readme and license files to extensions.
  fixed the "only" option for FileHelper::findFiles()
  Added Query::indexBy
  Improved FileHelper::filterPath()
  Enhanced file options for FileHelper.
  Fixed locale command.
typo fix in migration docs
Fixed matching all case.
Fixes #622.