📝 Add loop.parent example

[homersimpsons]

The current wording does not give enough explanation to me. I had to find https://stackoverflow.com/questions/17534802/how-does-loop-parent-actually-work to understand its need.

I added an example with variable shadowing, should I add one with nested loops? such as:

{% for i in [1, 2, 3] %}
  {% for j in [1, 2, 3] %}
    {{ loop.parent.loop.index }} - {{ loop.index }}
  {% endfor %}
{% endfor %}

[homersimpsons]

(this can be a duplicate of #4862)

[fabpot]

Closing in favor of #4862