Add warning for client implementers (#74)

* Add warning for client implementers

From discussion with @yasharpm

* whitespace

* Use "MAY" instead of will

---------

Co-authored-by: Giuseppe Lo Presti <giuseppe.lopresti@cern.ch>

spec.yaml

@@ -423,6 +423,9 @@ definitions:
                 Otherwise, at least `webdav` is expected to be supported, and
                 its options MAY be given in the opaque `options` payload for
                 compatibility with v1.0 implementations (see examples).
+                CAREFUL: client implementers should be aware that v1.1 servers
+                MAY support both `webdav` and `multi`, but v1.0 servers MAY
+                only support `webdav`.
             options:
               type: object
               description: |