replace mention of 'hook_link' with a proper description

There is no hook_link() in D7; it was removed.

Therefore documentation that says 'the same format as hook_link()' is not very helpful now.

The mentions of 'hook_link' in the docs hook_flag_link() for should be updated, and the actual format explained in full detail.

AFAICT this only returns an array with title and href, at least in our implementation of it. Would be good to check what the module code does with what it gets from the invocation of that hook, just to be sure what its effective spec is!