Enhance Javadoc descriptions

- added Javadoc to all the public methods
- fixed inconsistent indents
- added a blank line between Javadoc header and @param section
Merge branch 'master' of github.com:recruit-tech/document-validator into enchance-javadoc-message
Merge pull request #77 from takahi-i/enchance-javadoc-message

Enchance javadoc