Clarify documentation of avoid channels expectations

Clarify documentation of avoid channels expectations

The vendor command QCA_NL80211_VENDOR_SUBCMD_AVOID_FREQUENCY was defined
to carry the list of avoid frequencies that aim to avoid any
interference with other coexistencies. This recommendation was followed
strictly by trying to prevent WLAN traffic on the impacted channels.

This commit refines the expectation of the interface by defining this
avoid channel list to allow minimal traffic but not heavier one. For
example, P2P may still be able to use avoid list frequencies for P2P
discovery and GO negotiation if the actual group can be set up on a not
impact channel.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>