Rollup merge of #100953 - joshtriplett:write-docs, r=Mark-Simulacrum

Rollup merge of #100953 - joshtriplett:write-docs, r=Mark-Simulacrum

Update documentation for `write!` and `writeln!`

https://github.com/rust-lang/rust/pull/37472 added this documentation, but it
needs updating:

- Remove some documentation duplicated between `writeln!` and `write!`
- Update `write!` docs: can now import traits as `_` to avoid conflicts
- Expand example to show how to implement qualified trait names