vdir: add references to CalDAV and CardDAV

Author: WhyNotHugo

content/specs/vdir.md

@@ -65,8 +65,11 @@ choose to store additional metadata in that filename, however, at the same time
 they MUST NOT assume that the metadata they included will be preserved by other
 programs.
 
-Filenames SHOULD be URL-safe, such that can be retained when synchronising to
-WebDAV collections.
+Filenames SHOULD be URL-safe, such that may be retained when synchronising to
+CalDAV (see: [RFC4791]) or CardDAV (see: [RFC6352]) collections.
+
+[RFC4791]: https://www.rfc-editor.org/rfc/rfc4791
+[RFC6352]: https://www.rfc-editor.org/rfc/rfc6352
 
 # Metadata