Minor tweaks to spillbuf naming and docstrings. No functional change.

* subversion/libsvn_subr/spillbuf.c
  (svn_spillbuf_t::spill_all_contents): Extend docstring to make it clearer
   that as long as all data fit into memory, no spill file will be created.
  (svn_spillbuf_t::filename): Renamed from svn_spillbuf_t::temp_path.
  (svn_spillbuf__get_filename, svn_spillbuf__write): Reflect the rename.
  (svn_spillbuf__write): Do not check if we have a memory buffer; it's redundant.
   Explain why we don't just discard the in-memory buffer if all contents were
   spilled to a file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1510435 13f79535-47bb-0310-9956-ffa450edef68