Minor Haddock markup improvement to Data.Monoid

[skip ci]
Refactor Haddock comments in Data.Bits

This removes the redundant "Minimal complete definition"-block included
in the Haddock comment since Haddock renders the `MINIMAL`-pragma as
well (which has is moved to the start of `class` definition for better
readability of the source code)

Morever, the references to `testBitDefault`, `bitDefault`, and
`popCountDefault` have been moved to the respective methods' Haddock
strings for which they can be used.
Fix lost Haddock annotation for `class Monad m`

This was broken in d94de87252d0fe2ae97341d186b03a2fbe136b04 when `join`
was inserted between `Monad`'s Haddock string and the `class Monad m`
definition thereby breaking the association.