HPCC-11031 Document MAXLENGTH Options

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>

@ghalliday - isn't it that adding MAXLENGTH uses the true (calculated) max length and therefore incompatible with older versions. Where as the default is to use the old default of 4K .. ?
See  issue : https://track.hpccsystems.com/browse/HPCC-11333