menuconfig: Add show-help mode

Update menuconfig (and Kconfiglib, just to sync) to upstream revision
e629c33d31e22, to add the commit below.

  menuconfig: Add show-help mode

  Pressing F toggles show-help mode, where the help window at the bottom
  displays the help text of the currently selected item, if any. Can be
  handy when browsing through symbols.

  Also mention the different modes that are available in the module
  docstring.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>