URL-SYNTAX: update IDNA section for WHATWG spec changes

WHATWG URL has dictated the use of Nontransitional Processing (IDNA
2008) for several years now. Chrome (and derivatives) still use
Transitional Processing, but Firefox and Safari have both switched.

Also document the fact that winidn functions differently from libidn2
here.

Closes #7026

docs/URL-SYNTAX.md

@@ -173,10 +173,14 @@ brackets). For example:
 If curl was built with International Domain Name (IDN) support, it can also
 handle host names using non-ASCII characters.
 
-curl supports IDN host names using the IDNA 2008 standard. This differs from
-browsers that follow the WHATWG URL spec, which dictates IDNA 2003 to be used.
-The two standards have a huge overlap but differ slightly, perhaps most
-famously in how they deal with the German "double s" (`ß`).
+When built with libidn2, curl uses the IDNA 2008 standard. This is equivalent
+to the WHATWG URL spec, but differs from certain browsers that use IDNA 2003
+Transitional Processing. The two standards have a huge overlap but differ
+slightly, perhaps most famously in how they deal with the German "double s"
+(`ß`).
+
+When winidn is used, curl uses IDNA 2003 Transitional Processing, like the rest
+of Windows.
 
 ## Port number