Currently, the Tool Options tab for most tools correctly shows the modifiers that can be used to change some of the options.  For example, the options for Blur/Sharpen, Dodge/Burn or Zoom show that Ctrl can be used to toggle the mode of the tool.  Similarly, the tooltips for the Mode of all selection tools explain the usage of Shift, Ctrl or Shift+Ctrl.

The options for the Path Tool (vectors tool) do not explain that Ctrl can be used to switch to Edit mode and Alt can be used to switch to Move mode.

In theory, it should be simple to replace:
  o Design
  o Edit
  o Move
by:
  o Design
  o Edit (Ctrl)
  o Move (Alt)

However, looking at app/tools/gimpvectoroptions.c:69, this is not so simple in practice because these options are generated by GIMP_CONFIG_INSTALL_PROP_ENUM(), which takes the strings associated with the enum and does not allow additional help messages to be supplied.  So either these tool options should be rewritten without using the convenience function, or a new function should be added, allowing extra help strings to be supplied for the options.