Clarify doc strings.

* subversion/include/svn_io.h
  (svn_io_open_uniquely_named): Clarify that the 'delete_when'
    parameter doesn't only have an effect when its value is 'on close',
    and that 'on pool cleanup' refers to the result pool.
  (svn_io_open_unique_file3,
   svn_stream_open_unique): Clarify that 'delete on pool cleanup'
    refers to the result pool.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1480426 13f79535-47bb-0310-9956-ffa450edef68