docs: faq: Updates to extend_with_default() example

[stuarteberg]

The FAQ explains that one could (in theory) write a schema in which default values aren't valid under their own schema, and then goes on to present some example code that would allow such a use-case.

Fair enough, but I think most users would rather have an example that actually does validate the defaults. Plus, if the default values are added to the instance before validation, then entire objects can be provided with default contents, even if the object was missing from the instance entirely. (The updated FAQ text in this PR provides an example.)

[stuarteberg]

In case it's helpful, you can view a pretty version of the new doc text here:
http://stuarteberg.github.io/jsonschema/faq.html#why-doesn-t-my-schema-that-has-a-default-property-actually-set-the-default-on-my-instance

[Julian]

Cool, thanks! Doc improvements are very much appreciated. I haven't read through this yet, but FWIW, I really don't recommend doing default setting during validation at all -- I forget whether that's mentioned in that part of the FAQ.

But I'll have a read through, thanks again!

[stuarteberg]

FWIW, I really don't recommend doing default setting during validation at all

Indeed, your text mentions that, and I didn't change it (I only added text). I know default-setting isn't great for all use-cases, but I still really appreciate that you gave an example in the FAQ. My only problem with the example code was that it didn't work with nested objects out-of-the-box. By swapping a few lines, now it does.

[Julian]

Cool, looks good, appreciated.