io: update documentation on LimitedReader

Specify that that LimitedReader returns EOF when the underlying
R returns EOF even if bytes remaining, N > 0.

Fixes #18271

Change-Id: I990a7135f1d31488d535238ae061d42ee96bacb7
Reviewed-on: https://go-review.googlesource.com/34249Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/io/io.go

@@ -420,6 +420,7 @@ func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} }
 // A LimitedReader reads from R but limits the amount of
 // data returned to just N bytes. Each call to Read
 // updates N to reflect the new amount remaining.
+// Read returns EOF when N <= 0 or when the underlying R returns EOF.
 type LimitedReader struct {
 	R Reader // underlying reader
 	N int64  // max bytes remaining