I updated this in the previous commit (https://golang.org/cl/17931)
but noticed a typo. and it still wasn't great.

The Go 1.5 text was too brief to know how to use it:

    // Trailer maps trailer keys to values, in the same
    // format as the header.

Change-Id: I33c49b6a4a7a3596735a4cc7865ad625809da900
Reviewed-on: https://go-review.googlesource.com/17932Reviewed-by: Russ Cox <rsc@golang.org>