update some App framework docs

Dependencies

• [2] U7M4M2F7 bugfix: don't rely on Screen_bottom1 while scrolling
• [3] AVTNUQYR basic test-enabled framework
• [4] 3QNOKBFM beginnings of a test harness
• [5] CA5T33FG one more implication
• [6] YTSPVDZH first successful pagedown test, first bug found by test

Change contents

• edit in app.lua at line 91
  [3.4318]

  [3.4318]
  + -- -- define a screen with room for 2 lines of text
• replacement in app.lua at line 96
  [3.4389]→[3.4389:4406](∅→∅)
  − -- App.font{
  [3.4389]

  [3.4406]
  + -- App.font.init{
• edit in app.lua at line 99
  [3.4434]

  [2.1133]
  + -- -- check that screen shows next 2 lines of text after hitting pagedown
• replacement in app.lua at line 101
  [2.1175]→[3.4475:4552](∅→∅),[3.2885]→[3.4475:4552](∅→∅),[3.4475]→[3.4475:4552](∅→∅)
  − -- App.check_screen_contents{

  − -- y0='ghi'

  − -- y15=''

  − -- }
  [2.1175]

  [3.4552]
  + -- App.screen.check(0, 'ghi')

  + -- App.screen.check(15, 'jkl')