Blender Git Loki

Making a few tooltips more descriptive, in light of recent confusion over the
purpose of some of these features

* ClampTo Constraint, Target Object field: now mentions that it is for curve
objects only
* Dopesheet Editor Modes: now describe what data is editable in each mode. The
wording may be a bit clumsy here, but (apart from Mask Editor one, which I'm not
entirely sure about) should at least be technically correct