Add an example with computed getter/setter to the 'Using v-model with components' section

[NataliaTepluhina]

We have a great example of v-model + computed getter/setter usage on Vuex docs. It would be nice to show similar example when using v-model on the component and on underlying input

computed: {
  message: {
    get () {
      return this.modelValue
    },
    set (value) {
      this.$emit('update:modelValue', value)
    }
  }
}

[NataliaTepluhina]

Closing an issue as related PR was merged