[documentation] Hide friend declarations in doxygen output

The clutter the output.  I can't believe anyone really wants to see
the friend relationships in doxygen.

doc/doxygen/Doxystyle

@@ -187,7 +187,7 @@ EXTRACT_LOCAL_METHODS  = NO
 EXTRACT_ANON_NSPACES   = NO
 HIDE_UNDOC_MEMBERS     = NO
 HIDE_UNDOC_CLASSES     = NO
-HIDE_FRIEND_COMPOUNDS  = NO
+HIDE_FRIEND_COMPOUNDS  = YES
 HIDE_IN_BODY_DOCS      = NO
 INTERNAL_DOCS          = NO
 CASE_SENSE_NAMES       = NO