[man] Describe 'cancel' action

man/man1/fzf.1

@@ -497,7 +497,7 @@ triggered whenever the query string is changed.
     \fBbackward-kill-word\fR    \fIalt-bs\fR
     \fBbackward-word\fR         \fIalt-b   shift-left\fR
     \fBbeginning-of-line\fR     \fIctrl-a  home\fR
-    \fBcancel\fR
+    \fBcancel\fR                (clears query string if not empty, aborts fzf otherwise)
     \fBclear-screen\fR          \fIctrl-l\fR
     \fBdelete-char\fR           \fIdel\fR
     \fBdelete-char/eof\fR       \fIctrl-d\fR