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

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
I would make the wording stronger:  

A function that takes a string parameter should also have the type prefixed by "const" so that modern compilers don't report errors when constant strings are passed to the function.  This may become the default in a future version.  (HPCC-12350)

Also "const string", "const unicode" etc. should also be added to the table above to make it clearer.  E.g.,

CONST STRING xyz           size32_t lenXyz, const char * xyz