Rollup merge of #40934 - SamWhited:improve_write_writeln_docs, r=steveklabnik
Rollup merge of #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 #29329, #29381

GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust