protocol: Suggest get_registry not be called frequently

I've heard some complaints that wl_display.get_registry "leaks"
server memory because wl_registry has no destructor.  While this isn't
strictly true - all those resources are freed when the client
disconnects - it's a bit of a gotcha for neophytes.

Since wl_registry's version is not requested in any way through
wl_display.get_registry, we can't add a destructor request without
breaking ABI.  So let's be a little more clear about the result
of getting too many wl_registry objects.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>