Document possible `io::ErrorKind`s of `fs::open`
Document possible `io::ErrorKind`s of `fs::open`

Try to make clear that this isn't an API guarantee for now, as we likely
want to refine these errors in the future, e.g. `ENOSPC` "No space left
on device".

CC #40322

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