Add docs for Debug* structs. #29355

Add docs for Debug* structs. #29355

This adds docs for the Debug* structs as well as examples from the
Formatter::debug_* methods, so that a user knows how to construct them.

I added these examples as the builders module is not public and hence
the debug_*_new() functions are not available to a user.

r? @steveklabnik

Review comments.

Mainly adding in the links for all of the structs and functions.

Remove rust tag on code blocks.