net/http: fix doc comment on PostFormValue function

This function checks Request.PostForm, which now includes values parsed
from a PATCH request.

Change-Id: I5d0af58d9c0e9111d4e822c45f0fb1f511bbf0d5
Reviewed-on: https://go-review.googlesource.com/114009Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/request.go

@@ -1254,8 +1254,8 @@ func (r *Request) FormValue(key string) string {
 	return ""
 }
 
-// PostFormValue returns the first value for the named component of the POST
-// or PUT request body. URL query parameters are ignored.
+// PostFormValue returns the first value for the named component of the POST,
+// PATCH, or PUT request body. URL query parameters are ignored.
 // PostFormValue calls ParseMultipartForm and ParseForm if necessary and ignores
 // any errors returned by these functions.
 // If key is not present, PostFormValue returns the empty string.