Move the NAN_KEY_ND_TK/ND_GTK/NM_TK/ND_IGTK/ND_BIGTK selectors from #defines
into a nan_key_type_t enum, and finalize their doc wording (drop the
"provisional" note now that the IGTK/BIGTK values are verified against the
blob ABI). They are still passed to esp_wifi_set_nan_key_internal() as the
int key_flag argument, so no call-site or ABI change. Also tidy the
surrounding doc comments.
8b2b386825