Minor comment update

[weierophinney]

The statement in injectEventManager's comments that a shared event manager is injected into an existing event manager is incorrect - the code replaces the existing event manager with a new one. That's what I updated.

I may have added too much 'how it works' detail, but I puzzled over this a bit so I thought the explanation might help future readers.

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Originally posted by @bitwombat at zendframework/zend-mvc#285

[weierophinney]

Also, I wonder about the pre-existing comment that "the conditional will always pass". I think that's true. Then does it need to be a conditional?

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Originally posted by @bitwombat at zendframework/zend-mvc#285 (comment)

[alexmerlin]

Closing issue due to being inactive for more than 1 year.