docs($injector): Reword `fn` param docs and link to DI information

The documentation for $injector.invoke is currently very unclear, and contains a typographic error.

I'd like to link to the more detailed DI docs instead of simply stating "function annotation" as though that means something out of context.

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**note**: I'm not sure if I need to specify this link as an external link or not. If it's possible, please let me know so I can correct this.