net80211: document some of the crypto/key functions

This documents the following functions:

• ieee80211_is_key_global()
• ieee80211_is_key_unicast()
• ieee80211_crypto_get_key_wepidx()
• ieee80211_crypto_get_keyid()
• ieee80211_crypto_get_txkey()
• ieee80211_crypto_encap()
• ieee80211_crypto_decap()

Differential Revision: https://reviews.freebsd.org/D52649