Auto merge of #43252 - vbrandl:doc/default-values, r=GuillaumeGomez

Document default values for primitive types

All primitive types implement the `Default` trait but the documentation just says `Returns the "default value" for a type.` and doesn't give a hint about the actual default value. I think it would be good to document the default values in a proper way.
I changed the `default_impl` macro to accept a doc string as a third parameter and use this string to overwrite the documentation of `default()` for each primitive type.
The generated documentation now looks like this:
![Documentation of default() on the bool primitive](https://i.imgur.com/nK6TApo.png)

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