Opt-out of processing rustdoc comments

[itegulov]

Thanks for the amazing library!

I know that there is a way to override docs on each individual struct field like this:

#[derive(JsonSchema)]
pub struct OverrideDocs {

    /// This comment will not be used as the description
    #[schemars(description = "This is the description!")]
    pub my_int: i32,

    /// This field will have no description set
    #[schemars(description = "")]
    pub my_undocumented_bool: bool,
}

But for my use case, it would be awesome if I could opt-out of processing rustdoc comments globally by enabling/disabling an option in SchemaSettings as was mentioned in #13 (comment).

If you are open to including such a feature, I would love to prepare a PR.