Add doc that received url parameter must be a valid URL

The `new URL(url)` statement will throw an error if the supplied
string is not a valid URL.  Adding a note to the function to clarify
that as requested by @Rob--W.

apply-css/background.js

@@ -27,6 +27,7 @@ function toggleCSS(tab) {
 
 /*
 Returns true only if the URL's protocol is in APPLICABLE_PROTOCOLS.
+Argument url must be a valid URL string.
 */
 function protocolIsApplicable(url) {
   const protocol = (new URL(url)).protocol;