base: Customary type class laws (e.g. for Eq) and non-abiding instances (e.g. Float) should be documented

As beginning Haskellers regularly ask about these laws and instances I think it would be good to have them documented where they are defined.

Documented so far:

(in 793902e6)

Classes

• Eq
• Floating
• Fractional
• Integral
• Num
• Ord

Non-abiding instances

• CDouble (shares Double's deficiencies)
• CFloat (shares Float's deficiencies)
• Complex a (inherits deficiencies)
• Double: Eq, Ord, Fractional, Num
• Float: Eq, Ord, Fractional, Num
• Ratio a (inherits deficiencies)
• Natural: Num

TODO

(This is not an exhaustive list, please add more)

• Document non-abiding instances for the types from Data.Word and Data.Int.