See attached patch.

Once patch goes through, I will submit a second part which updates
regs.texi.

Some context:

I usually to use M-x append-to-register (and with this patch `C-x r +')
to collect sexps.  I think it will be convenient to have the collected
snippets separated by a suitable separator. Is it OK if a default "\n"
be used? WDYT.  Note that this will be a departure from current
behaviour where the snippets are blindly concatenated.

ps: Patch should apply cleanly to below bzr version.

,----
| kjambunathan@debian-6:~/src/emacs/trunk$ bzr version-info
| revision-id: rgm@gnu.org-20120817072823-0zjgqf10g1d0vcf1
| date: 2012-08-17 00:28:23 -0700
| build-date: 2012-08-17 15:17:41 +0530
| revno: 109655
| branch-nick: trunk
`----