Since this documentation is confusing, maybe these notes will help.
Shape keys are linked to the mesh data.
~ Do not modify the number of vertices.
~ Add basis in object mode.
~ Add them in object mode, edit them in edit mode to set their geometry.
~ Operate on the basis key only, if possible. If you operate on a different key, trust blender; just look through your keys to make sure all their values are 0 and do not try to to reëdit any of them.
~ Do not worry if on the first tab into object mode no changes can be seen. It is currently set at 0%
~ Do not worry when switching to a key. The selected key is shown at 100% for clarity and overrides key value and any IPOs. Switch the frame or key value to reset.
~ At each keyframe, set each slider to the desired value. This can be done in the Buttons Window or IPO Curve Editor, but it is better to do it in the Action Editor. Use the IPO Curve Editor to modify the transitions between shapes.
~ Follow 1-3 in the above.
~ Try to only operate on the basis key.
~ Do not worry when switching to a key. The selected key is shown at 100% for clarity and overides key value and any IPOs. Switch the frame to reset.
~ Same keyframing technique as with relative.