Clarify purpose of helper.InjectPluginVolume(mutatedPod) call in lifecycle example

In the lifecycle example code there's a call to `helper.InjectPluginVolume(mutatedPod)`

This appears to inject a volume into the postgres Pod being managed.

It's unclear from the code why this is done, what use cases it might serve, and what the injected volume actually is. Is it meant to be shared state (a cluster filesystem volume?) between all pods? An empty PV? A scratch data area for the plugin?

It would help to clarify this in both the code where it's invoked and in the doc-comment on the helper method.

Start at the lifecycle example call to helper.InjectPluginVolume(mutatedPod), then read the helper method's doc-comment and implementation. Confirm what volume is injected and why, including its intended use cases, and update both the invocation context and helper documentation so the behavior is understandable.

Written by the indexing model from the issue text.