io: clarify SeekEnd offset value

fixes #53474

Change-Id: I14c3dc800dc27233630a54592328bb0df1bbaa5d
GitHub-Last-Rev: 46f93cf
GitHub-Pull-Request: #53505
Reviewed-on: https://go-review.googlesource.com/c/go/+/413614
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Emmanuel Odeke <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: howjmay

src/io/io.go

@@ -111,7 +111,8 @@ type Closer interface {
 // 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.
+// SeekEnd means relative to the end
+// (for example, offset = -2 specifies the penultimate byte of the file).
 // Seek returns the new offset relative to the start of the
 // file or an error, if any.
 //