Docs: Improve IFF() description

The description for IFF() from the 7.0.8 language reference manual reads:

The IFF function performs the same functionality as IF, but ensures that an expression containing complex boolean logic is evaluated exactly as it appears.

What does that really mean? Does it mean that IF() evaluates its expression in a way that is not exactly how it appears?

It would be helpful if there were some examples of where you would use IFF() instead of IF().