Rollup merge of #85288 - Geal:clarify-std-io-read, r=dtolnay
Rollup merge of #85288 - Geal:clarify-std-io-read, r=dtolnay

add an example to explain std::io::Read::read returning 0 in some cases

I have always found the explanation about `Read::read` returning 0 to indicate EOF but not indefinitely, so here's more info using Linux as example. I can also add example code if necessary

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