Add a note that if an error is returned after having read
at least the minimum no. of bytes, the error is set to nil.

Fixes #20477

Change-Id: I75ba5ee967be3ff80249e40d459da4afeeb53463
Reviewed-on: https://go-review.googlesource.com/102459Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>