Add half-page-{up,down} actions (#784)

man/man1/fzf.1

@@ -470,6 +470,8 @@ e.g. \fBfzf --bind=ctrl-j:accept,ctrl-k:kill-line\fR
     \fBnext-history\fR          (\fIctrl-n\fR on \fB--history\fR)
     \fBpage-down\fR             \fIpgdn\fR
     \fBpage-up\fR               \fIpgup\fR
+    \fBhalf-page-down\fR
+    \fBhalf-page-up\fR
     \fBpreview-down\fR
     \fBpreview-up\fR
     \fBpreview-page-down\fR