Clarify why it must not be used in a irq handler

[MatiasVara]

documentation/Networking.md

Line 62 in 0468f3a

* This function is neither thread safe nor interruption safe. For this reason, it must not be used in the context of an interruption handler. The cooperative scheduler ensures non-preemption. That said, it is impossible to have two instances of the function in a different thread in the same core.