It's pretty simple really:
1. if you are not in the hand you can't say anything, including what you folded or if the flop would have connected w you.
2. if it's heads up, you can say almost anything.
3. if there are more than two people remaining in the hand you can't say anything that would give undue advantage
to another opponent. This would be the case if player A raises, with players B and C to act an C saying he was going to fold (let's B know that he wont face a raise behind, can't count on C being in the pot to give him odds
, etc.); also, for example, B can't say what his hand is because it would give C information that he didn't have previously. However, player A can say something like "I'm bluffing" as it doesn't necessarily give either of the remaining players and advantage, even though it gives them (both equally) additional information.
4. tournaments are slightly different than ring games in that even players who are not in the hand have a vested interest in the outcome of the hand, so the rules are a bit stricter.
5. imo, the "you can't tell the truth about your hand" rule is horrible. Steve Z. wrote a nice article on why here
; basically, if you have to lie then you are revealing the truth anyway by definition.