Add tag push events to the API

A few months back you added push events on tags. However, you cannot set the tag URL via the API.

This adds the tag events as a valid API parameter and updates the documentation accordingly.

Without this patch, if you try to add the "tag_push_events" hook, it falls into the system hooks and essentially does nothing.

From what I can tell in the code, this might just be an oversight from when the tag events were added in the first place since it took very little to enable this functionality.
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