Rollup merge of #55111 - Havvy:rustc-book-cfg-examples, r=GuillaumeGomez
Rollup merge of #55111 - Havvy:rustc-book-cfg-examples, r=GuillaumeGomez

[Rustc Book] Explain --cfg's arguments

I removed this from the reference since it's rustc specific, and noticed it wasn't well documented on the page that should document it well. It does seem to go against the grain of one line per command line flag though.

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