Improve DistantCopCarSirens documentation

[barbeque-squared]

The distant sirens when any player had their emergency lights on (regardless of whether the vehicle actually has a siren) are annoying. At first I was unsure of whether this native would fix that problem (the description reads like it's just on/off) but after some testing it turns out:

• the default is true
• when set to true, it will only actually play the sound when any player has emergency lights on. If no-one has emergency lights on, the sound isn't played.
• when toggling it off while the sound is playing, the distant sirens actually fade out, and vice versa.

Usecase: we have a race grid of about 30 cars, plus 1 safety car which has emergency lights (only lights, no actual siren). All players except the safety car player and the vehicles very near to it would hear constant or intermittent distant sirens. Setting it to false has resolved this issue.

NB: I'm not sure if this also affects NPC sirens. I only tested it with actual players.
NB2: feel free to add more commits before merging if you want to reword things.