HPCC-11046 Include IFF in contents of ECLLanguageReference

Separate out IFF from the description of IF in the language reference manual.

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Merge pull request #5556 from jamienoss/issue11046-IFF-in_langref

HPCC-11046 Include IFF in contents of ECLLanguageReference

Reviewed-By: Jim DeFabia <james.defabia@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>