Flesh out use of field fallback_value

[ltickett]

See #2783

I have tried to shoehorn the word / term default value in as that is what I was searching for when trying to find this funcitonality.

I have also fleshed out the usage with one of the examples.

I hope this is helpful.

[ltickett]

@rmosolgo i've had a stab- feel free to close if you don't believe it adds value

Thanks

[rmosolgo]

Sorry I let this go dormant. I'm open to improving the docs on this option if you'd like to open a new PR with docs-only changes!