message: docs: add half-page scroll shortcut to docs
diff: @@ -56,6 +56,8 @@ Note that key bindings are generally case-sensitive.
  | ++g+g++ , ++home++ | Jump to the first entry |
  | ++G++ , ++end++ | Jump to the last entry |
  | ++page-up++ , ++page-down++ | Scroll up/down a table by a page |
  +| ++ctrl+u++ | Scroll up a table by half a page |
  +| ++ctrl+d++ | Scroll down a table by half a page |
  ## Mouse bindings
  