net/http: document that ServeMux.Handler can also synthetize a 405

Also, fix a minor typo in ServeMux.Handle and ServeMux.HandleFunc.

Change-Id: I6a6a46565719104cb8f2484daf0e39f35b55a078
Reviewed-on: https://go-review.googlesource.com/c/go/+/675835
Reviewed-by: David Chase <[email protected]>
Auto-Submit: Filippo Valsorda <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: FiloSottile

src/net/http/server.go

@@ -2674,7 +2674,8 @@ func stripHostPort(h string) string {
 // the path that will match after following the redirect.
 //
 // If there is no registered handler that applies to the request,
-// Handler returns a “page not found” handler and an empty pattern.
+// Handler returns a “page not found” or “method not supported”
+// handler and an empty pattern.
 //
 // Handler does not modify its argument. In particular, it does not
 // populate named path wildcards, so r.PathValue will always return
@@ -2864,7 +2865,7 @@ func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
 // always refers to user code.
 
 // Handle registers the handler for the given pattern.
-// If the given pattern conflicts, with one that is already registered, Handle
+// If the given pattern conflicts with one that is already registered, Handle
 // panics.
 func (mux *ServeMux) Handle(pattern string, handler Handler) {
 	if use121 {
@@ -2875,7 +2876,7 @@ func (mux *ServeMux) Handle(pattern string, handler Handler) {
 }
 
 // HandleFunc registers the handler function for the given pattern.
-// If the given pattern conflicts, with one that is already registered, HandleFunc
+// If the given pattern conflicts with one that is already registered, HandleFunc
 // panics.
 func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
 	if use121 {