HPCC-9877 Clarify use of const in BEGINC++ docs

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
HPCC-9877 Edits based on review feedback

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
Merge pull request #6500 from JamesDeFabia/9877CPP_pointers

HPCC-9877 Clarify use of const in BEGINC++ docs

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>