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>