Add chapter explaining what Linux terminal is

DESIGN.rst

@@ -39,11 +39,10 @@ http://doryen.eptalys.net/libtcod/
 
 
 
-Platform specific notes
------------------------
-Linux doesn't have a concept of `console`, because there
-is another historical concept of `terminal`. There are many
-terminal types and the closest one to `console` is `tty`.
+Platform specific terminology
+-----------------------------
+Linux doesn't have a concept of `console`. Thing that plays
+the role of the console on Linux is called `text terminal`.
 
 For the sake of clarity the `console` here is a window with
 defined width and height in characters.
@@ -55,7 +54,57 @@ with the help of file descriptior, which doesn't have width
 and height properties. To overcome this limitation Linux
 has a set of special calls named I/O control calls (ioctl).
 Given file descriptor, it is possible to query system for
-additional information associated with it.
+additional information associated with it, or change
+behavior of the attached terminal.
+
+
+Understanding Linux terminal
+----------------------------
+Terminal under Linux is a complicated thing to get right
+without a proper entrypoint. Good starter is Wikipedia
+article about text terminals and especially paragraph about
+ `dumb terminals`.
+http://en.wikipedia.org/wiki/Text_terminal#Text_terminals
+
+For completeteness, I post some relevant info here. Text
+terminal is keyboard + display. Keyboard is used to type
+input and display to show some stuff::
+
+    +-------------------+
+    |   Text terminal   |
+    |   +-----------+            +-----------+
+    |   |           |  #0 stdin  |           |
+    |   | Keyboard  +----------->|  Program  |
+    |   |           |   |        |           |
+    |   +-----------+   |        +---+---+---+
+    |                                |   |
+    |   +-----------+    #1 stdout   |   |
+    |   |           |<---------------+   |
+    |   |  Display  |      #2 stderr     |
+    |   |           |<-------------------+
+    |   +-----------+   |
+    +-------------------+
+
+This picture looks like your program filters input and
+decide what will be sent to display. Not quite right. The
+need to show input on the screen immediately was so
+frequent that terminal devices do this themselves, and to
+turn this feature off you have to configure them.
+
+Configuration is done through calls to Linux "terminal
+input/output system" (termios) with #0, #1 and #2 file
+descriptors. This is a common Linux API to change terminal
+behavior.
+
+Turning off canonical mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+Some terminals allowed users to prepare line before sending
+it to the program. Users were able to edit line with cursor
+and backspace keys, and keyboard <-> display interaction
+was done completely inside terminal. Program received the
+input only after user hit `enter` button. This was called
+`canonical mode`. `pager` turns off canonical mode, because
+it needs every single keypress as soon as it occurs.
 
 
 Getting console size on Linux