HPCC-8880 TOXML not documented in ECL Language Reference

Signed-off-by: Richard Taylor <richard.taylor@lexisnexis.com>
Merge pull request #4304 from RichardTaylorHPCC/B8880

HPCC-8880 TOXML not documented in ECL Language Reference

Reviewed-By: Richard Chapman <rchapman@hpccsystem.com>