Sync'ing FSX with FSFS:
Porting documentation and iterpool usage improvments.
There are no specific revision that were merged by this.

* subversion/libsvn_fs_x/index.c
  (encode_uint,
   encode_int,
   encode_l2p_page,
   l2p_page_table_access_func,
   get_l2p_page_table): Update function docstring.
  (prefetch_l2p_pages): Ditto. Also, clear iterpools at loop start.
  (l2p_index_lookup): Clear iterpools at loop start.
  (svn_fs_x__l2p_get_max_ids): Update commentary.
  (svn_fs_x__p2l_index_create): Same and clear iterpools at loop start.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1597199 13f79535-47bb-0310-9956-ffa450edef68