Add a note to our documentation about plan branch trigger inheritance

Issue Summary

It'd be great if we could add a small note to the Using plan branches – Branch triggers documentation that if you don't change the trigger type for your plan branch by default they will be inherited from the main branch configuration. You don't have to go there and choose to use the same triggers that are defined in the parent plan – this is already happening if you choose not to change the trigger for that plan branch.

I believe adding perhaps "(default)" next to the first option would already be helpful:

• Same as defined in parent plan (default)
• None. Run new plan branches manually
• Custom trigger