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

library/core/src/macros/mod.rs

@@ -457,11 +457,12 @@ macro_rules! r#try {
 ///
 /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
 /// implementing either, as objects do not typically implement both. However, the module must
-/// import the traits qualified so their names do not conflict:
+/// avoid conflict between the trait names, such as by importing them as `_` or otherwise renaming
+/// them:
 ///
 /// ```
-/// use std::fmt::Write as FmtWrite;
-/// use std::io::Write as IoWrite;
+/// use std::fmt::Write as _;
+/// use std::io::Write as _;
 ///
 /// fn main() -> Result<(), Box<dyn std::error::Error>> {
 ///     let mut s = String::new();
@@ -474,6 +475,23 @@ macro_rules! r#try {
 /// }
 /// ```
 ///
+/// If you also need the trait names themselves, such as to implement one or both on your types,
+/// import the containing module and then name them with a prefix:
+///
+/// ```
+/// # #![allow(unused_imports)]
+/// use std::fmt::{self, Write as _};
+/// use std::io::{self, Write as _};
+///
+/// struct Example;
+///
+/// impl fmt::Write for Example {
+///     fn write_str(&mut self, _s: &str) -> core::fmt::Result {
+///          unimplemented!();
+///     }
+/// }
+/// ```
+///
 /// Note: This macro can be used in `no_std` setups as well.
 /// In a `no_std` setup you are responsible for the implementation details of the components.
 ///
@@ -526,25 +544,6 @@ macro_rules! write {
 ///     Ok(())
 /// }
 /// ```
-///
-/// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
-/// implementing either, as objects do not typically implement both. However, the module must
-/// import the traits qualified so their names do not conflict:
-///
-/// ```
-/// use std::fmt::Write as FmtWrite;
-/// use std::io::Write as IoWrite;
-///
-/// fn main() -> Result<(), Box<dyn std::error::Error>> {
-///     let mut s = String::new();
-///     let mut v = Vec::new();
-///
-///     writeln!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
-///     writeln!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
-///     assert_eq!(v, b"s = \"abc 123\\n\"\n");
-///     Ok(())
-/// }
-/// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "writeln_macro")]