Rollup merge of #53299 - MagnumOpus21:fix-macro-write, r=steveklabnik

Updated core/macros.rs to note it works in a no_std environment.

Fixes #45797
This PR updates the documentation of `write!` to note it works in a `no_std` environment, and adds an
example to showcase this. In a `no_std` environment, the author of the code is responsible for the
implementation of the `Write` trait. This example will work out of the box with `no_std`, but the
implementation of `Write` is expected to be provided by the user.

r? @steveklabnik