kubectl: rewrite docstrings in several files

Fixing inaccuracies and clarifying in the case of ambiguities.

pkg/kubectl/proxy_server.go

@@ -108,7 +108,8 @@ func (f *FilterServer) accept(method, path, host string) bool {
 	return false
 }
 
-// Make a copy of f which passes requests along to the new delegate.
+// HandlerFor makes a shallow copy of f which passes its requests along to the
+// new delegate.
 func (f *FilterServer) HandlerFor(delegate http.Handler) *FilterServer {
 	f2 := *f
 	f2.delegate = delegate