Rollup merge of #31610 - Manishearth:doc-clarify-txrx, r=steveklabnik
Rollup merge of #31610 - Manishearth:doc-clarify-txrx, r=steveklabnik

Not everyone knows this convention. We could just rename the variables in the
example, but since this notation is commonly used it's a good opportunity to
introduce it.

r? @steveklabnik

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