io/fs: clarify that "." may only be used for root

For #70155

Change-Id: I648791c484e19bb12c6e4f84e2dc42eaaa4db546
Reviewed-on: https://go-review.googlesource.com/c/go/+/624595
Reviewed-by: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: ianlancetaylor

src/io/fs/fs.go

@@ -44,7 +44,7 @@ type FS interface {
 // Path names passed to open are UTF-8-encoded,
 // unrooted, slash-separated sequences of path elements, like “x/y/z”.
 // Path names must not contain an element that is “.” or “..” or the empty string,
-// except for the special case that the root directory is named “.”.
+// except for the special case that the name "." may be used for the root directory.
 // Paths must not start or end with a slash: “/x” and “x/” are invalid.
 //
 // Note that paths are slash-separated on all systems, even Windows.