Add a better help message for kubectl describe

When resource type isn't given in the command, print the possible
resource types. Also added the resource type description in help
strings.

docs/man/man1/kubectl-describe.1

@@ -26,6 +26,12 @@ $ kubectl describe RESOURCE NAME\_PREFIX
 will first check for an exact match on RESOURCE and NAME\_PREFIX. If no such resource
 exists, it will output details for every resource that has a name prefixed with NAME\_PREFIX
 
+.PP
+Possible resources include pods (po), replicationcontrollers (rc), services
+(svc), nodes (no), events (ev), componentstatuses (cs), limitRanges (limits),
+persistentVolumes (pv), persistentVolumeClaims (pvc), resourceQuotas (quota)
+or secrets.
+
 
 .SH OPTIONS
 .PP