The gzip package is asymmetrical in the way it handles headers.
In Writer, the Header is written on the first call to Write, Flush, or Close.
In Reader, the Header is read on calls to NewReader or Reset as opposed to
after the first Read. Thus, we document this difference.

Fixes #13211

Change-Id: I5f87beff036e5e2fd68a02a15fdb7137e9ca4c37
Reviewed-on: https://go-review.googlesource.com/16838Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>