add help docs and ?

docs/nodes/Histogram.md

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+# Histogram
+
+Compute the height distribution histogram (DH). Use log scale to reveal small peaks next to a dominant background. Outputs marker measurements while showing the histogram interactively in-node. Equivalent to gwy_data_field_dh.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| measurements | RECORD_TABLE | Measurements at the two cursor markers (x/y positions and dx/dy) |
+| marker_pair | COORDPAIR | Current cursor marker positions |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| n_bins | INT | 256 | Number of histogram bins (10–1000) |
+| y_scale | dropdown | linear | Y-axis scale: linear or log |
+
+## Limitations
+
+- Cursor positions are stored as fractions of the histogram range and are set interactively.
+- Log scale displays bins as log(count); bins with zero count appear as the minimum log value.