net/http: try to document ServeFile security more

We've expanded this several times. Try more.

Fixes #18837

Change-Id: I03b699391351a30ee60a15d7aa712c6c66444cf9
Reviewed-on: https://go-review.googlesource.com/125875Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/fs.go

@@ -650,15 +650,23 @@ func localRedirect(w ResponseWriter, r *Request, newPath string) {
 // file or directory.
 //
 // If the provided file or directory name is a relative path, it is
-// interpreted relative to the current directory and may ascend to parent
-// directories. If the provided name is constructed from user input, it
-// should be sanitized before calling ServeFile. As a precaution, ServeFile
-// will reject requests where r.URL.Path contains a ".." path element.
+// interpreted relative to the current directory and may ascend to
+// parent directories. If the provided name is constructed from user
+// input, it should be sanitized before calling ServeFile.
 //
-// As a special case, ServeFile redirects any request where r.URL.Path
+// As a precaution, ServeFile will reject requests where r.URL.Path
+// contains a ".." path element; this protects against callers who
+// might unsafely use filepath.Join on r.URL.Path without sanitizing
+// it and then use that filepath.Join result as the name argument.
+//
+// As another special case, ServeFile redirects any request where r.URL.Path
 // ends in "/index.html" to the same path, without the final
 // "index.html". To avoid such redirects either modify the path or
 // use ServeContent.
+//
+// Outside of those two special cases, ServeFile does not use
+// r.URL.Path for selecting the file or directory to serve; only the
+// file or directory provided in the name argument is used.
 func ServeFile(w ResponseWriter, r *Request, name string) {
 	if containsDotDot(r.URL.Path) {
 		// Too many programs use r.URL.Path to construct the argument to