- Removes a subject-verb disagreement.
- Documents that PATCH requests also populate PostForm.
- Explains that r.PostForm is always set (but blank for GET etc.).

Fixes #16609

Change-Id: I6b4693f8eb6db7c66fd9b9cd1df8927f50d46d50
Reviewed-on: https://go-review.googlesource.com/32091Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>