Fix description for Valid_Speed_Up and Valid_Speed_Down

[asineth0]

Valid_Speed_Up and Valid_Speed_Down actually control the maximum allowed speed on the Y-axis and not the forward/reverse speed for each vehicle movement packet.

This can be a bit confusing for mod developers wondering why cars might be teleporting back when going up hills, which is one of the common pain points with vehicles lagging on servers.

Most lag going up hills can be fixed just by increasing Valid_Speed_Up and Valid_Speed_Down, without sacrificing much in terms of movement validation.

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