Add Javadoc comments for public user registration strategies.

• info.magnolia.module.publicuserregistration.strategy.Mail - Sends the registered public user a verification email which contains a link. When the user clicks the link, the system enables the user.
• info.magnolia.module.publicuserregistration.strategy.Never - Does not enable the user. An administration needs to enable the user manually by selecting the Enabled checkbox.
• info.magnolia.module.publicuserregistration.strategy.Always - Enables the registered public user immediately. No verification needed.

While at it fix also checkstyle.