document bindparam() vs. literal()

[Mike Bayer]

I'm seeing a lot of SO answers come out doing this:

select([some_value).<someop> <something else>](bindparam('some_name',))

when they should be using literal(). Let's beef up the docs for bindparam() and literal() to be fully cross-referenced and explain the difference. Let's also make this clear in BindParam, that it really has these two public constructor functions.