docs(js): Further clarify denyUrls (#13481)

Author: chargome

includes/platforms/configuration/options/deny-urls.mdx

@@ -3,4 +3,6 @@ Errors that have been created on these URLs won't be sent to Sentry.
 If you use this option, errors will not be sent when the top stack frame file URL contains or matches at least one entry in the `denyUrls` array.
 All string entries in the array will be matched with `stackFrameUrl.contains(entry)`, while all RegEx entries will be matched with `stackFrameUrl.match(entry)`.
 
+This option checks the source file URL in the stack trace, not the HTTP URL where the error was reported. For more fine grained filtering please refer to [beforeSend](/platforms/javascript/guides/react/configuration/options/#beforeSend).
+
 This matching logic applies to captured exceptions not raw message events. By default, all errors are sent.