Update the documentation of custom modifiers to specify the need of quotes in return type

[sharadregoti]

Can we update the documentation of custom modifiers, to clearly specify that return type string should have quotes. If it doesn't have quotes we get empty value as result

package main

import (
	"strings"

	"github.com/tidwall/gjson"
)

const json = `{"name":{"first":"Janet","last":"Prichard"},"age":47}`

func main() {
	gjson.AddModifier("case", func(json, arg string) string {
		if arg == "upper" {
			return strings.ToUpper(json)
		}
		if arg == "lower" {
			return strings.ToLower(json)
		}
		return json
	})
	gjson.AddModifier("age", func(json, arg string) string {
		if arg == "0" {
                        // Quotes is necessary here
			return `"test"`
		}
		return json
	})

	value2 := gjson.Get(json, "name.last|@case:upper")
	println(value2.String())
	value := gjson.Get(json, "name.last|@age:0")
	println(value.String())
}

I am thinking of adding this as note below custom modifier doc section

Please note that returned string type should have double quotes in it. 
	gjson.AddModifier("age", func(json, arg string) string {
		if arg == "custom-arg" {
                        // Quotes is necessary here
			return `"test"`
		}
		return json
	})