Final touches to the FSFS low-level API implementation:
Update docstrings and change pool names of internal functions.

No functional change.

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__read_changes_incrementally): Correct pool name in docstring.

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__read_changes_incrementally): Use the same pool name as in
                                           the declaration.
  (write_change_entry): The pool is actually a scratch pool.
  (read_header_block,
   svn_fs_fs__read_noderev,
   format_digest): The pools are actually result pools.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605073 13f79535-47bb-0310-9956-ffa450edef68