gnrc_netreg: Further specify gnrc_netreg_register() precondition
gnrc_netreg: Further specify gnrc_netreg_register() precondition

While browsing through the rendered doc, I found the precondition of
`gnrc_netreg_register()` somewhat lacking. What is a "message queue"?
`gnrc_netreg_entry_t`s have types. Does this apply for all types?
This specifies the requirement more: The calling thread **only** needs
a message queue (also provides a link to `msg_init_queue()` now for
further information), if the provided `gnrc_netreg_entry_t` is of type
`GNRC_NETREG_TYPE_DEFAULT` (i.e. thread-wise IPC).

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