Rename and document a parameter of svn_wc_parse_externals_description3().

The docstring did not explain the meaning of the 'parent_directory' parameter,
it merely stated how it is used internally.
The name 'parent_directory' is misleading since the parameter is supposed
to be the path or URL to the directory on which the svn:externals property
corresponding to the externals description is set. This property is not
necessarily set on the direct parent directory of the external, it can be
set on any parent. Usually the term 'defining directory' is used in this
context, so rename the parametr to 'defining_directory'.

* subversion/include/svn_wc.h
  (svn_wc_parse_externals_description3): Rename 'parent_directory' to
    'defining_directory'.

* subversion/libsvn_wc/externals.c
  (svn_wc_parse_externals_description3): Rename 'parent_directory' to
    'defining_directory' and 'parent_directory_display' to
    'defining_directory_display'. No functional change.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1642690 13f79535-47bb-0310-9956-ffa450edef68