Core/Arena: Fix an exploit which made you gain personal rating when you leave arena, and add some comments to the arena system (which I should have done way earlier).

--HG--
branch : trunk

src/server/game/Battlegrounds/ArenaTeam.cpp

@@ -644,6 +644,7 @@ int32 ArenaTeam::GetRatingMod(uint32 own_rating, uint32 enemy_rating, bool won,
 
 int32 ArenaTeam::GetPersonalRatingMod(int32 base_rating, uint32 own_rating, uint32 enemy_rating)
 {
+    // max (2 * team rating gain/loss), min 0 gain/loss
     float chance = GetChanceAgainst(own_rating, enemy_rating);
     chance *= 2.0f;
     return (int32)ceil(float(base_rating) * chance);
@@ -671,6 +672,7 @@ void ArenaTeam::FinishGame(int32 mod)
 int32 ArenaTeam::WonAgainst(uint32 againstRating)
 {
     // called when the team has won
+    // own team rating versus opponents matchmaker rating
     int32 mod = GetRatingMod(m_stats.rating, againstRating, true);
 
     // modify the team stats accordingly
@@ -685,6 +687,7 @@ int32 ArenaTeam::WonAgainst(uint32 againstRating)
 int32 ArenaTeam::LostAgainst(uint32 againstRating)
 {
     // called when the team has lost
+    // own team rating versus opponents matchmaker rating
     int32 mod = GetRatingMod(m_stats.rating, againstRating, false);
 
     // modify the team stats accordingly