docs: add an example for slots components with props

[hiendv]

Because many people including me (:rofl:) were struggling to mount slots components with props, this PR introduces an example for the case.
Example:

<!-- Component.vue -->
<template>
  <div><slot/></div>
</template>

<!-- Child.vue -->
<template>
  <p>{{ foo }}</p>
</template>
<script>
export default {
  props: {
    foo: {
      type: String,
      required: true
    }
  }
}
</script>

<!-- Expected mounting -->
<component>
  <child foo="bar"></child>
  <child foo="qux"></child>
</component>

Btw, I don't really know if this fits into the api docs or common tips.

[afontcu]

LGTM 👍 I think it would be "too much" to add this to the Common tips section, as this is not a common scenario – as far as I can tell!

[JessicaSachs]

Just needs a little more context to explain to the developer what kind of Components this will help test. Everything looks great implementation-wise ✅

[JessicaSachs]

💯Super! Thank you.

[vue-bot]

Hey @hiendv, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚