Where is Grease Pencil stroke snap mode in UI? I found this: gpencil_stroke_snap_mode

gpencil_stroke_snap_mode
NONE All points, No snap.
ENDS End points, Snap to first and last points and interpolate.
FIRST First point, Snap
to first point.

But i can't find this settings in user interface. Where is it?
Thank you for posting your question very clearly.
This is the reason why SNAP FIRST and SNAP LAST and LOOSE SNAP exist.
This was recently implemented.
When you START a drawing using the circle snap your drawing will conform to the first and last stroke point when you let go of the draw. If you draw and CLOSE the shape the snap will conform to first point to snap. And obviously if you don´t do any of these, you´ll have LOOSE snap. 

I´m glad that you´re reading into this inside the API.
Thank you for your vote for this answer if you find what you were looking for

BFCT_Schiller