plugins: Document lack of .cache in remaining language bindings
lua, perl, python, and ruby have more work before they can support
.cache callbacks (for that matter, we still haven't implemented
generic v2 interface support there, starting with can_fua).
The rust bindings are at least at version 2, and don't mention missing
bindings in the .pod page. However, they do not yet have extents
support, which should be added before doing the obvious copy-and-paste
to enable cache support, so it is not touched here.
Signed-off-by: Eric Blake <eblake@redhat.com>