Add caveat about passing callbacks using local fun references

Callback funs provided in apns:connect/3 end up as arguments in the ``apns_sup`` child spec for starting ``apns_connection`` children.

If you load new code for the module where the original callback fun was taken from, and it was a local fun reference, the code server will kill the ``apns_sup`` as an orphan process for holding on to an old version of the code it needs to purge.