Rollup merge of #29029 - steveklabnik:iter_function_docs, r=alexcrichton
Rollup merge of #29029 - steveklabnik:iter_function_docs, r=alexcrichton

Flesh the docs out with examples.

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