* subversion/include/private/svn_named_atomic.h
  (svn_named_atomic__is_efficient): Add docstring note about memory
                                    consumption. No functional change.

Suggested by: brane


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1613152 13f79535-47bb-0310-9956-ffa450edef68