add section about default commands

docs/docs/intro.mdx

@@ -240,6 +240,13 @@ $('body').terminal({
 
 If you type `h` and press `<TAB>` it will complete the command and insert `ello`.
 
+## Default Commands
+
+When you use completion and press `<TAB>` twice, without typing anything, you will have list of
+commands.  By default you will also have `clear` command that you can disable with `clear: false`
+option. There is also second default command which is exit, that appear only when you use
+[authentication](#???) or [nested interpreters](#???).
+
 ## Demo
 
 import CodePen from '@site/src/CodePen';