[jTEI] add support for cross-references to notes

[rvdb]

The jTEI ODD file describes how cross-references can be made to <div>, <figure>, and <table> elements. Another (as yet unsupported) common target for cross-references are footnotes in <note> elements. Support can be added easily by

• modifying the documentation prose for cross-references
• extending the ISO Schematron rule checking for valid cross-reference targets

These changes complement the changes required to the TEI Stylesheets reported in https://github.com/TEIC/Stylesheets/issues/165.

I'll issue a pull request suggesting the necessary changes.

[hcayless]

Done