Update playbooks_error_handling.rst

[S1ructure]

Add example for using own variables in conditions

Related: https://forum.ansible.com/t/using-variables-in-changed-when-failed-when-directives/10652/3

From my point of view this information was not clear yet, that
a) custom variables can be used in conditionals as well
b) the {{ }} annotation has to be avoided

[oraNod]

Thanks very much for adding this to the docs @S1ructure - I've made a few suggestions. I'd also wonder if this should go into the variables section of the docs, maybe here: https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_variables.html#referencing-simple-variables

As I mentioned in one of my suggestions, I think putting this in the error handling section focuses too much on resolving a warning rather than promoting the best way to do something, if that makes sense.

[oraNod]

I don't think we should include the exact warning text here. Granted this might be helpful to users who search for the warning but it adds maintenance overhead to the docs. I also don't believe it's good practice to document errors and warnings - better to document just the behaviour.

[oraNod]

Suggested change

	Just like ``when`` these two conditionals do not require templating delimiters (``{{ }}``) as they are implied.
	Just like ``when`` these two conditionals do not require templating delimiters (``{{ }}``) because they are raw Jinja2 expressions.

[oraNod]

Maybe we could add a link to this section of the docs: https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_conditionals.html#basic-conditionals-with-when

[oraNod]

Suggested change

	If you want to introduce your own variables, to avoid repeating a certain term, you can simply reference them in your conditionals
	You can reference simple variables in conditionals to avoid repeating certain terms, as in the following example:

[oraNod]

Suggested change

	Notice the missing ``{{ }}`` around log_path.
	Notice the missing double curly braces ``{{ }}`` around the ``log_path`` variable in the ``changed_when`` statement.