libs: drop some public APIs.

libs: drop some public APIs.

Don't expose GstVaapiContext APIs and make them totally private to
libgstvaapi core library. That API would also tend to disappear in
a future revision. Likewise, don't expose GstVaapiDisplayCache API
but keep symbols visible so that the various render backends could
share a common display cache implementation in libgstvaapi.

Try to clean-up the documentation from any stale entry too.