Add a bunch of summary documentation for the spill-buffer type.
Add a bunch of summary documentation for the spill-buffer type.

* subversion/include/private/svn_subr_private.h:
  (): add a @defgroup for the spill buffer and some more docco

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion