Document EntityPlayer.GetActiveSubCharge

[Zamiell]

I don't know exactly what it does, but it should be documented that:

• It is nit related to the Battery. This function always returns 0, regardless of what the Battery charge it. You get the Battery charge by using EntityPlayer.GetBatteryCharge.
• It is not related to 4.5 Volt. When the player has 4.5 volt, the EntityPlayer.GetCharge method works as expected. There is not a way to get the "partial" charge towards the next tick of charge. This function always returns 0, regardless of what the partial charge is.
• It is not related to the Schoolbag. This function always returns 0, regardless of the charge of the Schoolbag item. You can get the Schoolbag item charge by using the EntityPlayer.GetCharge method with the correct ActiveSlot argument.

[wofsauge]

Added info in 7cd3bb0