HPCC-9408 Document changes to the way MODULE inheritances works

Signed-off-by: Richard Taylor <richard.taylor@lexisnexis.com>
Merge pull request #4460 from RichardTaylorHPCC/B9408

HPCC-9408 Document changes to the way MODULE inheritances works

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>