add help docs and ?

docs/nodes/Polynomial Level.md

@@ -0,0 +1,28 @@
+# Polynomial Level
+
+Fit and subtract a polynomial background of given degree in x and y. Equivalent to gwy_data_field_fit_polynom.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to level |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| leveled | DATA_FIELD | Field with polynomial background subtracted |
+| background | DATA_FIELD | The fitted polynomial background surface |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| degree_x | INT | 2 | Polynomial degree in the x direction (0–5) |
+| degree_y | INT | 2 | Polynomial degree in the y direction (0–5) |
+
+## Limitations
+
+- High polynomial degrees (> 4) may overfit and introduce artificial long-range modulation.
+- No masking support; all pixels contribute equally to the fit.