Improve comments in the config around {low,high,max}_temp settings

[hyperair]

Amend the comments in mbpfan.conf around low_temp, high_temp, and max_temp to convey more information about what these settings actually are. The meaning was derived from the comments in this region:

mbpfan/src/mbpfan.c

Lines 57 to 59 in 52d8973

	* low_temp - temperature below which fan speed will be at minimum
	* high_temp - fan will increase speed when higher than this temperature
	* max_temp - fan will run at full speed above this temperature */

Fixes: #203

[gaul]

Thank you for your contribution @hyperair!

[hyperair]

You're welcome! :)