wpa_priv: Document reduced functionality

wpa_priv has never really been fully up-to-date with the wpa_supplicant
driver interface extensions. This does not seem like something that
would change in the future either, so document this reduced
functionality as a potential drawback.

Signed-off-by: Jouni Malinen <j@w1.fi>

wpa_supplicant/README

@@ -965,6 +965,17 @@ wpa_priv can control multiple interface with one process, but it is
 also possible to run multiple wpa_priv processes at the same time, if
 desired.
 
+It should be noted that the interface used between wpa_supplicant and
+wpa_priv does not include all the capabilities of the wpa_supplicant
+driver interface and at times, this interface lacks update especially
+for recent addition. Consequently, use of wpa_priv does come with the
+price of somewhat reduced available functionality. The next section
+describing how wpa_supplicant can be used with reduced privileges
+without having to handle the complexity of separate wpa_priv. While that
+approve does not provide separation for network admin capabilities, it
+does allow other root privileges to be dropped without the drawbacks of
+the wpa_priv process.
+
 
 Linux capabilities instead of privileged process
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