syntax: Fix a missing closing code tag in docs.
syntax: make deriving have slightly less cryptic error messages.

This unfortunately changes an error like

    error: mismatched types: expected `&&NotClone` but found `&NotClone`

into

    error: type `NotClone` does not implement any method in scope named `clone`
auto merge of #11826 : huonw/rust/7621-deriving-errors, r=alexcrichton

cc #7621.

See the commit message. I'm not sure if we should merge this now, or wait until we can write `Clone::clone(x)` which will directly solve the above issue with perfect error messages.