Add an option to disable alternative screens.

st.1

@@ -3,6 +3,7 @@
 st \- simple terminal
 .SH SYNOPSIS
 .B st
+.RB [ \-a ]
 .RB [ \-c
 .IR class ]
 .RB [ \-f
@@ -23,6 +24,9 @@ st \- simple terminal
 is a simple terminal emulator.
 .SH OPTIONS
 .TP
+.B \-a
+disable alternate screens in terminal
+.TP
 .BI \-c " class"
 defines the window class (default $TERM).
 .TP