On the fsfs-format7 branch: Document the new pack code.
While at it, tweak minor implementation details.

* subversion/libsvn_fs_fs/pack.c
  (): overview to the pack process
  (rep_info_t,
   pack_context_t): document data structs
  (initialize_pack_context,
   reset_pack_context,
   close_pack_context,
   copy_file_data,
   write_null_bytes,
   copy_item_to_temp,
   get_item_array_index,
   add_item_rep_mapping,
   copy_rep_to_temp,
   svn_fs_fs__order_dir_entries,
   copy_node_to_temp,
   compare_p2l_info,
   sort_items,
   compare_p2l_info_rev,
   sort_by_rev,
   pick_recursively,
   sort_reps,
   copy_items_from_temp,
   append_entries,
   write_l2p_index,
   append_revision): add docstrings and code comments 
  (pack_section): document and rename to ...
  (pack_range): ... this
  (pack_log_addressed): update caller; document
  (pack_phys_addressed): drop FS parameter; document
  (pack_rev_shard): update caller


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1451128 13f79535-47bb-0310-9956-ffa450edef68