Rollup merge of rust-lang#40934 - SamWhited:improve_write_writeln_docs, r=steveklabnik

Improve the docs for the write and writeln macros

This change reduces duplication by linking the documentation for
`writeln!` to `write!`. It also restructures the `write!` documentation
to read in a more logical manner (I hope; feedback would be welcome).

Updates rust-lang#29329, rust-lang#29381

Author: frewsxcv

src/libcore/macros.rs

@@ -337,27 +337,20 @@ macro_rules! try {
 
 /// Write formatted data into a buffer
 ///
-/// This macro accepts a 'writer' (any value with a `write_fmt` method), a format string, and a
-/// list of arguments to format.
+/// This macro accepts a format string, a list of arguments, and a 'writer'. Arguments will be
+/// formatted according to the specified format string and the result will be passed to the writer.
+/// The writer may be any value with a `write_fmt` method; generally this comes from an
+/// implementation of either the [`std::fmt::Write`] or the [`std::io::Write`] trait. The macro
+/// returns whatever the 'write_fmt' method returns; commonly a [`std::fmt::Result`], or an
+/// [`io::Result`].
 ///
-/// The `write_fmt` method usually comes from an implementation of [`std::fmt::Write`][fmt_write]
-/// or [`std::io::Write`][io_write] traits. The term 'writer' refers to an implementation of one of
-/// these two traits.
+/// See [`std::fmt`] for more information on the format string syntax.
 ///
-/// Passed arguments will be formatted according to the specified format string and the resulting
-/// string will be passed to the writer.
-///
-/// See [`std::fmt`][fmt] for more information on format syntax.
-///
-/// `write!` returns whatever the 'write_fmt' method returns.
-///
-/// Common return values include: [`fmt::Result`][fmt_result], [`io::Result`][io_result]
-///
-/// [fmt]: ../std/fmt/index.html
-/// [fmt_write]: ../std/fmt/trait.Write.html
-/// [io_write]: ../std/io/trait.Write.html
-/// [fmt_result]: ../std/fmt/type.Result.html
-/// [io_result]: ../std/io/type.Result.html
+/// [`std::fmt`]: ../std/fmt/index.html
+/// [`std::fmt::Write`]: ../std/fmt/trait.Write.html
+/// [`std::io::Write`]: ../std/io/trait.Write.html
+/// [`std::fmt::Result`]: ../std/fmt/type.Result.html
+/// [`io::Result`]: ../std/io/type.Result.html
 ///
 /// # Examples
 ///
@@ -396,27 +389,12 @@ macro_rules! write {
 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
 ///
-/// This macro accepts a 'writer' (any value with a `write_fmt` method), a format string, and a
-/// list of arguments to format.
-///
-/// The `write_fmt` method usually comes from an implementation of [`std::fmt::Write`][fmt_write]
-/// or [`std::io::Write`][io_write] traits. The term 'writer' refers to an implementation of one of
-/// these two traits.
-///
-/// Passed arguments will be formatted according to the specified format string and the resulting
-/// string will be passed to the writer, along with the appended newline.
-///
-/// See [`std::fmt`][fmt] for more information on format syntax.
-///
-/// `write!` returns whatever the 'write_fmt' method returns.
+/// For more information, see [`write!`]. For information on the format string syntax, see
+/// [`std::fmt`].
 ///
-/// Common return values include: [`fmt::Result`][fmt_result], [`io::Result`][io_result]
+/// [`write!`]: macro.write.html
+/// [`std::fmt`]: ../std/fmt/index.html
 ///
-/// [fmt]: ../std/fmt/index.html
-/// [fmt_write]: ../std/fmt/trait.Write.html
-/// [io_write]: ../std/io/trait.Write.html
-/// [fmt_result]: ../std/fmt/type.Result.html
-/// [io_result]: ../std/io/type.Result.html
 ///
 /// # Examples
 ///