io: use SeekStart, SeekCurrent, and SeekEnd in io.Seeker documentation

The documentation previously used C style enumerations: 0, 1, 2.
While this is pretty much universally correct, it does not help a user
become aware of the existence of the SeekStart, SeekCurrent, and SeekEnd
constants. Thus, we should use them in the documentation to direct people's
attention to them.

Updates #6885

Change-Id: I44b5e78d41601c68a0a1c96428c853df53981d52
Reviewed-on: https://go-review.googlesource.com/23551Reviewed-by: Andrew Gerrand <adg@golang.org>

src/io/io.go

@@ -102,10 +102,12 @@ type Closer interface {
 // Seeker is the interface that wraps the basic Seek method.
 //
 // Seek sets the offset for the next Read or Write to offset,
-// interpreted according to whence: 0 means relative to the start of
-// the file, 1 means relative to the current offset, and 2 means
-// relative to the end. Seek returns the new offset relative to the
-// start of the file and an error, if any.
+// interpreted according to whence:
+// SeekStart means relative to the start of the file,
+// SeekCurrent means relative to the current offset, and
+// SeekEnd means relative to the end.
+// Seek returns the new offset relative to the start of the
+// file and an error, if any.
 //
 // Seeking to an offset before the start of the file is an error.
 // Seeking to any positive offset is legal, but the behavior of subsequent