Fix issue #391: Add some documentation to indicate that a default constructor must be explicitly declared in a user-defined serializable type in order for it to be serializable.
Patch by: me
Review by: gwt.team.morrildl


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@437 8db76d5a-ed1c-0410-87a9-c151d255dfc7