Rollup merge of #29027 - steveklabnik:iter_docs, r=manishearth
Rollup merge of #29027 - steveklabnik:iter_docs, r=manishearth

Due to the way iterators work, the std::iter module is chock full of
structs that you, humble Rust programmer, will never actually use.
However, they have URLs, and therefore get linked to, because they are
real structs that do exist.

So, rather than just have a tiny sentence about each one of them,
this patch adds links back to the functions and methods on Iterator
which actually create the structs, where helpful documentation already
exists.

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