bundler: type must be of type itemExample

Add commentary about item type
https://github.com/googleapis/google-api-go-client/issues/422

Fixes #422

Change-Id: I344e5bdff7918c5bc3a4d35615a9426bece8c1b2
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/53390
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
diff --git a/support/bundler/bundler.go b/support/bundler/bundler.go
index 418143d..5f603bd 100644
--- a/support/bundler/bundler.go
+++ b/support/bundler/bundler.go
@@ -214,6 +214,8 @@
 
 // Add adds item to the current bundle. It marks the bundle for handling and
 // starts a new one if any of the thresholds or limits are exceeded.
+// The type of item must be assignable to the itemExample parameter of the NewBundler
+// method, otherwise there will be a panic.
 //
 // If the item's size exceeds the maximum bundle size (Bundler.BundleByteLimit), then
 // the item can never be handled. Add returns ErrOversizedItem in this case.