[READY] Better flexibility for abstract db types

MSSQL seems to support limits for decimal type, so we add a default
limit.

Merge branch 'querybuilder-dynamic-column-types' of github.com:cebe/yii2 into querybuilder-dynamic-column-types

* 'querybuilder-dynamic-column-types' of github.com:cebe/yii2:
  Added phpdoc about precision constraints for abstract db types

Feature was requested in yiisoft/yii#765

fixes #465

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