Dispatcher and @dispatch cannot be mixed - documentation should clearly state that

The documentation [design.rst](https://github.com/mrocklin/multipledispatch/master/docs/source/design.rst) should clearly state that one must either use the approach using `Dispatcher()` or the approach using `@dispatch`. These two cannot be mixed. Currently the example in **Namespaces and dispatch** merely states `# f = Dispatcher('f') # no need to create Dispatcher ahead of time` But actually in the sequence ``` f = Dispatcher('f') @f.register(int) def inc(x): return x + 1 @dispatch(float) def f(x): return x - 1 ``` the @dispatch overwrites `f` with a new Dispatcher object, and the registration `f(int)` is lost. This is by design: `@dispatch` uses the `namespace` to store names and definitions, while `Dispatcher` is unaware of `namespaces`.