find: document ! (not) operator

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/findutils/find.c b/findutils/find.c
index 0ec88fd..d49adc2 100644
--- a/findutils/find.c
+++ b/findutils/find.c
@@ -1067,6 +1067,9 @@
 //usage:	IF_FEATURE_FIND_PAREN(
 //usage:     "\n	( ACTIONS )	Group actions for -o / -a"
 //usage:	)
+//usage:	IF_FEATURE_FIND_NOT(
+//usage:     "\n	! ACT		Invert ACT's success/failure"
+//usage:	)
 //usage:     "\n	ACT1 [-a] ACT2	If ACT1 fails, stop, else do ACT2"
 //usage:     "\n	ACT1 -o ACT2	If ACT1 succeeds, stop, else do ACT2"
 //usage:     "\n			Note: -a has higher priority than -o"