add help docs and ?

docs/nodes/Watershed Segmentation.md

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+# Watershed Segmentation
+
+Segment a height field into grains using the two-stage Gwyddion watershed workflow: drop-based seed location followed by watershed growth. Supports hill or valley detection and optional union/intersection with an existing mask.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input topographic field |
+| mask | IMAGE | No | Optional existing mask to combine with the watershed result |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| mask | IMAGE | Binary grain mask from watershed segmentation |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| invert_height | BOOLEAN | False | When True, detects valleys instead of hills (inverts the height field) |
+| locate_steps | INT | 10 | Number of drop steps in the seed location stage (1–200) |
+| locate_threshold | INT | 10 | Minimum drop threshold for seed acceptance (0–100000) |
+| locate_drop_size | FLOAT | 0.1 | Relative drop size for seed location stage (0.0001–1.0) |
+| watershed_steps | INT | 20 | Number of steps in the watershed growth stage (1–2000) |
+| watershed_drop_size | FLOAT | 0.1 | Relative drop size for watershed growth stage (0.0001–1.0) |
+| combine_mode | dropdown | replace | How to combine with an existing mask: replace (ignore existing), union (OR), or intersection (AND) |
+
+## Limitations
+
+- Parameter tuning is required; the default settings work best for isolated round features.
+- Very large locate_threshold values may reject all seeds and produce an empty mask.