os: Improve File.Seek documentation

[bamiaux]

Currently, the documentation on os.File.Seek mentions:

Seek sets the offset for the next Read or Write on file to offset, interpreted according
to whence: 0 means relative to the origin of the file, 1 means relative to the current
offset, and 2 means relative to the end.

It leads people to write things like file.Seek(0, 2) which is in my opinion, not that
readable, especially when you don't know where those numbers come from.

I believe it would be better if the following os constants were mentioned instead:
 * SEEK_SET
 * SEEK_CUR
 * SEEK_END

[gopherbot]

Comment 1 by islandberry23:

See https://golang.org/issue/6873

[robpike]

Comment 2:

Status changed to Duplicate.

Merged into issue #6873.

[dominikh]

Comment 3:

This isn't precisely a duplicate. issue #6873 is concerned with moving the constants
around, this issue merely suggests making the existing ones more visible.