docs(derive): Add a few notes to default_value

[nicholasbishop]

• Clarify that the value is parsed as if it had been passed via command line, not as Rust code like some other libraries (e.g. argh).

• Add an example for u32, but note that default_value_t is better for that type.

• Mention that this attribute is a good way to set the default value for String args. This should help avoid confusion with trying to use default_value_t for that purpose (see How to stop warnings about parentheses required by clap macros? #4676 for example)

[epage]

This is reading more as general default_value documentation that we generally defer to the builder documentation. #4090 is about that relationship, even if this one is actually included due to defaulting.

I'd be inclined to close this.