Revision 1803546

Following up on r1803140, comment on why the memory usage of the new
svn_txdelta_to_svndiff_stream() function is limited, and on the fact
that we rely on the implementation detail of svn_txdelta_to_svndiff3().

See the related discussion in:
  https://lists.apache.org/thread.html/51c8418956457d2739e2672340fbcacc86d9ee40be7f8dae447e998d@%3Cdev.subversion.apache.org%3E

* subversion/libsvn_delta/svndiff.c
  (window_handler, svndiff_stream_write_fn, svn_txdelta_to_svndiff_stream):
  Add comments and cross-link them.