Disclaimer: I might be not completely accurate, but seem to be close enough to the truth.
The item 6c) in the list of 4.2 release notes says that everything that is a motor will use MOT_PWM_MIN and MOT_PWM_MAX.
Regarding SERVOx_FUNCTION, which assigns functions to outputs, “a motor” corresponds to, I believe, a list of functions which are known to be motors. If your function is one of these, it will use MOT_PWM_MIN and MOT_PWM_MAX. Otherwise, it will use the corresponding SERVOx_ limits.
Since you can re-assign functions to outputs in essentially whatever manner you like, SERVO5_PWM_MIN just sits and waits until you assign a non-motor function to the fifth output.