kubectl: rewrite docstrings in several files

Fixing inaccuracies and clarifying in the case of ambiguities.

pkg/kubectl/generate.go

@@ -35,7 +35,9 @@ type GeneratorParam struct {
 	Required bool
 }
 
-// Generator is an interface for things that can generate API objects from input parameters.
+// Generator is an interface for things that can generate API objects from input
+// parameters. One example is the "expose" generator that is capable of exposing
+// new replication controllers and services, among other things.
 type Generator interface {
 	// Generate creates an API object given a set of parameters
 	Generate(params map[string]interface{}) (runtime.Object, error)