vgacon: comment on vga_rolled_over

Long time ago, I figured out what this number is good for and documented
that locally. But never submitted, so do it now.

Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Jiri Slaby <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/video/console/vgacon.c

···
 static bool 		vga_512_chars;
 static int 		vga_video_font_height;
 static int 		vga_scan_lines		__read_mostly;
-static unsigned int 	vga_rolled_over;
+static unsigned int 	vga_rolled_over; /* last vc_origin offset before wrap */
 
 static bool vgacon_text_mode_force;
 static bool vga_hardscroll_enabled;