Try to fix up two WC-NG doc strings in update_editor.c.
Try to fix up two WC-NG doc strings in update_editor.c.

### Add an assertion.

* subversion/libsvn_wc/update_editor.c
  (accumulate_last_change): Write a better doc string.
  (install_added_props): Add docco about the function and how it's in
    transition to WC-NG.

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