What special rules do you tell your opponent before your game?

I always wonder what special rule to tell my opponent before the game. In a tournament setting I always ask them how well they know my army. Depending on their answer I either tell them a lot, or a little. 

Here is an example of what and when to tell them of special rules: My foot Eldar always run an Avatar, so when do I tell them that it is immune to melta and flamer weapons? Do I tell them before the game, before they shoot, or after they shoot? It seems realistic that the troops might never have fought an Avatar before and do not have the intelligence to know what can hurt it and what can't and it is just trial and error until they get it right in game. But it also seems fair to apprise my opponent what they can do.

I have played against many Necron players and I do not think that any of them have ever told me in any detail the rules for We Will Be Back. They might tell me that the model comes back on a roll of 4+, but they have never told me what makes them not get a roll at all. It would be a huge advantage to them if I don't know how to get around WBB ahead of time, and if I am told that they would lose that advantage. 

As everyone who is reading this should know, I am taking Demonhunters to Adepticon. The Demonhunters codex is so old, and the army is so rare that there are a lot of people who do not know the armies special rules. So what I am going to do is to help out my opponents and on the first page of all of my army lists I am going to have a cheat sheet detailing all of my special rules.

It should look something like this:

Grey Knight Special Rules

Shrouding: 3d6x3 inches to spot and target Grey Knight (Red) units.

Aegis: If you target a GK unit with a psychic power it acts like a Psychic Hood.

Grey Knight Psycannon: 3, Strength 6, AP 4 shots   Range: Assault 18”, Heavy 36” No invulnerable saves.

Inquisitor’s Rules

Mystics: 4d6 inches to shoot at a Deep Striking unit.

Captain Stern’s Rules

Not an Independent Character while his retinue is still alive

Force Weapon: Psychic test to instant kill models, and it ignores Eternal Warrior

Grimoire of True Names: Demons are at half weapon skill in B-t-B

The Strands of Fate: Can re-roll one dice roll per phase, but opponent gets a re-roll.

Psychic Powers-

Hammerhand-(doubles strength, but not a power weapon)

Holocaust- Strength5 AP4, large blast, 0” range.