Add warning callout re: bool and store_true defaults

[johnowhitaker]

Using bool arg with default True results in the default getting over-written (see e.g. #533) which has confused a few people. This PR adds a small warning to the docs to use bool_arg instead.

[johnowhitaker]

@jph00 it's a small thing but I've made this mistake in the past (setting my_param:bool=True not realizing it will still default to False). This PR adds a small callout to the docs with a warning to use bool_arg instead of bool in this case.
Demo of what I'm talking about: https://gist.github.com/johnowhitaker/52ae8cbeddaf43a62d345253f515ac97

[jph00]

Thank you!