Fix breakage with Order.min and Order.max on nightly

[KodrAus]

In rust-lang/rust#42496, min and max methods have been added to the Ord trait, which causes an issue using glm's BaseNum::min/max functions for a type that implements Ord. This was reported here

This PR currently just clarifies which min/max function to use in the docs to fix tests. It's probably not what you want long-term. If you require BaseNum to be compared, then you could add an Ord bound and remove the min and max methods.

[KodrAus]

cc: @dche

[dche]

Thanks!

BaseNum can not have Ord trait because f32 and f64 are not Eq. Since in the users' code the crate level functions glm::min/glm::max should always be used, your fix is completely OK.