ceph: expand doc to go a bit
Make the doc comment in doc.go a bit more fleshed out with pointers
to the real functionality of the library.
/*
-Set of wrappers around Ceph APIs.
+Package ceph is the root of a set of packages that wrap the Ceph APIs.
+
+Generally, this package only exists to host subsystem specific packages
+and contains no functions.
+
+The "rados" sub-package wraps APIs that handle RADOS specific functions.
+
+The "rdb" sub-package wraps APIs that handle RBD specific functions.
+
+The "cephfs" sub-package wraps APIs that handle CephFS specific functions.
+
+Consult the documentation for each package for additional details.
*/
package ceph
