io/fs: move path name documentation up to the package doc comment
Perhaps surprisingly to users, io/fs path names are slash-separated.
Move the documentation for path names up to the top of the package
rather than burying it in the ValidPath documentation.
Change-Id: Id338df07c74a16be74c687ac4c45e0513ee40a8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/708616
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>