Allow argument documentation

[bryanburgers]

Is your feature request related to a problem? Please describe.

I like to provide clear documentation to my end users about everything in the schema, and this includes arguments.

Describe the solution you'd like

As of Rust 1.39, attributes are allowed on parameters. rust-lang/rfcs#2565.

I would like to see these used to provide documentation for arguments.

#[juniper::object(Context = Context)]
impl QueryRoot {
    #[doc = "Ability to document the endpoint already exists"]
    fn some_endpoint(
        context: &Context,
        #[doc = "Ability to document arguments would be awesome"]
        id: ID,
        #[doc = "Only include items in one of these states. Defaults to all states."]
        states: Option<Vec<StateInput>>
    ) -> SomeObject {
        todo!()
    }
}

Describe alternatives you've considered

We don't need to use #[doc = ""]. We could also use a custom attribute.

Additional context

Documenting arguments is common. For example, on the GitHub v4 API, everything considered a "Connection" has its arguments clearly documented.

#334 seems to suggest that this works. However, I can't seem to get it to work. Possibly it doesn't work in #[juniper::object] macros?

[LegNeato]

I believe this will be enabled by #441, right @davidpdrsn ?

[davidpdrsn]

Not sure. I will add it as a todo to look into.

[Victor-Savu]

Using attributes to document parametrs would be great! However, it should not hold back #441. If I am not mistaken, it is a backwards-compatible feature, so it can be added in later without breaking any of the code that might already be relying on #441. @davidpdrsn you are the best parson to decide on that.