vt: document vc_data by example

All those members of vc_data are each explained in short. But it needs
an example for one to understand the whole picture.

So add an ascii art depicting the most important vc_data members.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/console_struct.h

@@ -21,6 +21,38 @@ struct uni_pagedir;
 
 #define NPAR 16
 
+/*
+ * Example: vc_data of a console that was scrolled 3 lines down.
+ *
+ *                              Console buffer
+ * vc_screenbuf ---------> +----------------------+-.
+ *                         | initializing W       |  \
+ *                         | initializing X       |   |
+ *                         | initializing Y       |    > scroll-back area
+ *                         | initializing Z       |   |
+ *                         |                      |  /
+ * vc_visible_origin ---> ^+----------------------+-:
+ * (changes by scroll)    || Welcome to linux     |  \
+ *                        ||                      |   |
+ *           vc_rows --->< | login: root          |   |  visible on console
+ *                        || password:            |    > (vc_screenbuf_size is
+ * vc_origin -----------> ||                      |   |   vc_size_row * vc_rows)
+ * (start when no scroll) || Last login: 12:28    |  /
+ *                        v+----------------------+-:
+ *                         | Have a lot of fun... |  \
+ * vc_pos -----------------|--------v             |   > scroll-front area
+ *                         | ~ # cat_             |  /
+ * vc_scr_end -----------> +----------------------+-:
+ * (vc_origin +            |                      |  \ EMPTY, to be filled by
+ *  vc_screenbuf_size)     |                      |  / vc_video_erase_char
+ *                         +----------------------+-'
+ *                         <---- 2 * vc_cols ----->
+ *                         <---- vc_size_row ----->
+ *
+ * Note that every character in the console buffer is accompanied with an
+ * attribute in the buffer right after the character. This is not depicted
+ * in the figure.
+ */
 struct vc_data {
 	struct tty_port port;			/* Upper level data */