add help docs and ?

docs/nodes/Tip Model.md

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+# Tip Model
+
+Generate a synthetic AFM tip model DATA_FIELD. The input field sets the pixel size for the tip. The apex (centre pixel) is the maximum value; edges are shifted to zero. Equivalent to gwy_tip_model_preset_create (tip.c / tip_model.c).
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Reference field that sets the tip pixel size |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| tip | DATA_FIELD | Synthetic tip shape field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| shape | dropdown | parabola | Tip geometry: parabola (paraboloid with apex radius R), cone (sphere-capped cone), or sphere (ball-on-stick) |
+| radius | FLOAT | 10 nm | Apex radius of curvature in metres (1e-10 to 1e-6) |
+| half_angle | FLOAT | 20.0 | Half-cone angle from the tip axis in degrees for the cone shape (1–89°) |
+| n_pixels | INT | 65 | Side length of the square tip grid in pixels (odd values only, 3–511) |
+
+## Limitations
+
+- half_angle is only used for the cone shape.
+- The tip grid must be smaller than or equal to the image size for Tip Deconvolution to work correctly.
+- Large n_pixels values produce large tip grids that make deconvolution slow.