How to autofocus an input in a Phoenix Form?

If you are writing a "traditional" Phoenix (without `LiveView`) App
and want to `autofocus` an `input` this works:

```html
<%= text_input f, autofocus: "true" %>
```

> **Note**: the _value_ of can be anything e.g: `autofocus: "autofocus"` works too as does `autofocus: ""` (empty string). We just prefer the value `"true"` for clarity in our code.

Whereas our 3 previous attempts _don't_ work:

```html
<%= text_input f, :autofocus %> # using an atom fails
```
```html
<%= text_input f, autofocus %> # using a keyword fails
```

```html
<%= text_input f, "autofocus" %> # using a string fails
```

We read through https://hexdocs.pm/phoenix_html/Phoenix.HTML.Form.html and did not find any reference to `autofocus` ... we read through https://elixirforum.com/t/phoenix-liveview-callbacks-to-put-text-input-focus-on-the-selected-input-field/21148 but that is specific to views rendered with `LiveView`.

If anyone is building a "traditional" Phoenix App as we are in https://github.com/dwyl/phoenix-todo-list-tutorial then this is the method that works.

No repository file, test, or entry point is named. Start by reviewing the Phoenix.HTML.Form documentation and the linked tutorial, then identify where guidance for traditional Phoenix form inputs belongs and consider the issue addressed when that guidance is captured in the project’s intended documentation.

Written by the indexing model from the issue text.