[Rd] Same class name, different package

[John Chambers]

In principle, two separately developed packages could use the same class 
name, and a user could then attach both and attempt to use methods for 
both classes.

That has never worked, but some changes have been added to r-devel to 
handle this case.  The changes involve extending the "signature" class 
to include package information.  For compatibility, packages will need 
to be re-installed from a version of R labelled 56466 or later, although 
an attempt is made to fill in missing information.

John