Fix feature gating

[tcharding]

We have feature gate issues in docs and with an unused error type.

I don't have a good way of including links that are feature build specific in rustdocs so I just used plain old back ticks.

[apoelstra]

Gonna go ahead and merge this. I'm not thrilled about the backtick thing but I don't know of any other solution.

[tcharding]

Yeah, encode and decode are literally the most important functions, not linking to them sucks.