Rollup merge of #37472 - joshtriplett:doc-fmt-write-io-write, r=brson

Document convention for using both fmt::Write and io::Write

Using a trait's methods (like `Write::write_fmt` as used in `writeln!` and other macros) requires importing that trait directly (not just the module containing it).  Both `fmt::Write` and `io::Write` provide compatible `Write::write_fmt` methods, and code can use `writeln!` and other macros on both an object implementing `fmt::Write` (such as a `String`) and an object implementing `io::Write` (such as `Stderr`).  However, importing both `Write` traits produces an error due to the name conflict.

The convention I've seen renames both of them on import, to `FmtWrite` and `IoWrite` respectively.  Document that convention in the Rust documentation for `write!` and `writeln!`, with examples.

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