cfg80211: add intro to documentation

This introduction section should be used in the documentation,
do that at the beginning of the cfg80211 chapter.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Documentation/driver-api/80211/cfg80211.rst

@@ -2,6 +2,9 @@
 cfg80211 subsystem
 ==================
 
+.. kernel-doc:: include/net/cfg80211.h
+   :doc: Introduction
+
 Device registration
 ===================