Add 'doc' argument to the 'xapr_index' method

* R/xapr_index.r (xapr_index): The 'doc' argument contains the
  character vector with data stored in the document.

* src/xapr.cpp (xapr_index): The 'doc' argument is passed to
  Xapian::Document set_data.

* man/xapr_index.Rd: Update documentation.