I've found some inconsistency between the manual and latest UI, details as below:
The stroke edit mode page says:
Enter Stroke Edit Mode with the Mode select menu in the 3D Views header or toggle the Enable Editing in the Grease Pencil panel, or use D-Tab. In this mode, many common editing tools will operate on Grease Pencil stroke points instead.
Screenshot is pasted here: (http://pasteall.org/pic/index.php?id=114499)
But the "Enable Editing" button is no longer available in Grease Pencel panel for Blender 2.78.
So, I want to know if it will be added back in later version. If the answer is "No", I think the manual should be corrected to avoid misleading users.