Add documentation for html_nested! macro

[AgarwalPragy]

This is about:

• Undocumented code

Problem
#843 introduces a macro html_nested! that adds support for iterable nested children access.

There is no documentation on yew.rs for the the html_nested! macro

---

Macro is currently hidden from docs

#[doc(hidden)]
#[proc_macro_hack(support_nested)]
pub use yew_macro::html_nested;

because

We prohibit public items without documentation by setting a crate attribute, so when this was added it was #[doc(hidden)] to allow the crate to compile.

---

Example - faulty code using html!

use wasm_bindgen::prelude::*;
use yew::prelude::*;

pub struct Container {
    link: ComponentLink<Self>,
    props: Props,
}

enum Msg {}

#[derive(Properties, Clone, PartialEq)]
pub struct Props {
    #[prop_or_default]
    pub children: ChildrenWithProps<Container>,
}

impl Component for Container {
    type Message = ();
    type Properties = Props;

    fn create(props: Self::Properties, link: ComponentLink<Self>) -> Self {
        Self { link, props }
    }

    fn update(&mut self, msg: Self::Message) -> ShouldRender {
        false
    }

    fn change(&mut self, _props: Self::Properties) -> ShouldRender {
        false
    }

    fn view(&self) -> Html {
        html! {
            <Container>
                {
                for (1..10_i32).map(|item| {
                    html! {
                        <Container/>
                    }
                })
                }
            </Container>
        }
    }
}

above code gives the error:

error[E0277]: the trait bound `yew::virtual_dom::VChild<example::Container>: std::convert::From<yew::virtual_dom::VNode>` is not satisfied
  --> webui/src/example.rs:37:21
   |
37 |                   for (1..10_i32).map(|item| {
   |  _____________________^
38 | |                     html! {
39 | |                         <Container/>
40 | |                     }
41 | |                 })
   | |__________________^ the trait `std::convert::From<yew::virtual_dom::VNode>` is not implemented for `yew::virtual_dom::VChild<example::Container>`
   |
   = note: required because of the requirements on the impl of `std::convert::Into<yew::virtual_dom::VChild<example::Container>>` for `yew::virtual_dom::VNode`
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

To fix, use html_nested!

    fn view(&self) -> Html {
        html! {
            <Container>
                {
                for (1..10_i32).map(|item| {
                    html_nested! {
                        <Container/>
                    }
                })
                }
            </Container>
        }
    }