Add description of Bound use in docs

[fonnesbeck]

The appropriate use of Bound is not clear, particularly since there is also a Bounded class. This can cause some confusion in usage (e.g. #1858). Would be good to have a subsection on this in the docs.

[fonnesbeck]

It occurs to me that we probably need an intermediate/advanced guide to complement our getting started guide.

[springcoil]

http://docs.pymc.io/api/bounds.html?highlight=bound -- does this resolve this?

[junpenglao]

yes that resolves it.