Add some new examples and instance documentation to base

I had to read the source for several things in base today, which I realized should have some more docs.

The instance documentation is on the instance itself, since it seems like Haddock can't document instance items as far as I can tell.

This adds:

• Documentation for the most nonobvious instances of Alternative/MonadPlus
• Example for msum, also clarify what the functions actually do (I think it is baffling that msum = asum and does not use mplus, however that is beyond the scope of this change).
• Example for (<$)