encoding/csv: document ReadAll behavior at EOF

Fixes #2847.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5641050

src/pkg/encoding/csv/reader.go

@@ -156,6 +156,9 @@ func (r *Reader) Read() (record []string, err error) {
 
 // ReadAll reads all the remaining records from r.
 // Each record is a slice of fields.
+// A successful call returns err == nil, not err == EOF. Because ReadAll is
+// defined to read until EOF, it does not treat end of file as an error to be
+// reported.
 func (r *Reader) ReadAll() (records [][]string, err error) {
 	for {
 		record, err := r.Read()