io: update documentation on LimitedReader

Specify that that LimitedReader returns EOF when the underlying
R returns EOF even if bytes remaining, N > 0.

Fixes #18271

Change-Id: I990a7135f1d31488d535238ae061d42ee96bacb7
Reviewed-on: https://go-review.googlesource.com/34249


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>