net/http: Improve docs for Response.ParseForm

- 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/32091
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/request.go

@@ -1078,18 +1078,24 @@ func parsePostForm(r *Request) (vs url.Values, err error) {
 	return
 }
 
-// ParseForm parses the raw query from the URL and updates r.Form.
+// ParseForm populates r.Form and r.PostForm.
 //
-// For POST or PUT requests, it also parses the request body as a form and
-// put the results into both r.PostForm and r.Form.
-// POST and PUT body parameters take precedence over URL query string values
-// in r.Form.
+// For all requests, ParseForm parses the raw query from the URL and updates
+// r.Form.
+//
+// For POST, PUT, and PATCH requests, it also parses the request body as a form
+// and puts the results into both r.PostForm and r.Form. Request body parameters
+// take precedence over URL query string values in r.Form.
+//
+// For other HTTP methods, or when the Content-Type is not
+// application/x-www-form-urlencoded, the request Body is not read, and
+// r.PostForm is initialized to a non-nil, empty value.
 //
 // If the request Body's size has not already been limited by MaxBytesReader,
 // the size is capped at 10MB.
 //
 // ParseMultipartForm calls ParseForm automatically.
-// It is idempotent.
+// ParseForm is idempotent.
 func (r *Request) ParseForm() error {
 	var err error
 	if r.PostForm == nil {