Fix docstrings and remove dead code across public API

Systematic audit of all public method docstrings against their
implementations, fixing mismatches and removing dead/broken code.

Docstring fixes:
- Fix `norm` type/default in render_shapes, render_points (was `bool | Normalize, default False`, now `Normalize | None`)
- Fix `palette` type across all render methods (add `dict[str, str]`)
- Remove false claim that palette defaults to lightgray when groups given
- Fix `cmap` description: continuous only, not discrete (render_shapes, render_points)
- Fix `scale` description in render_shapes: applies to polygons too
- Fix `method` description: "or" not "and", add >10k threshold
- Fix `show()` return type: `Axes | list[Axes] | None`, not `sd.SpatialData`
- Document all 13 missing `show()` parameters
- Fix `render_labels` return type (was `None`)
- Fix `render_points` return saying "shapes" (copy-paste)
- Fix `render_labels` broadcasting example referencing render_images
- Add Notes section to render_images: RGB auto-detection, compositing, cmap broadcast
- Add type annotations to colorbar/colorbar_params across all methods
- Fix `make_palette_from_data` color column lookup description
- Fix OutlineParams docstring (was "Cmap params")
- Fix typo "precendence" → "precedence"

Dead code removal:
- Remove `share_extent` from `show()` (accepted but never used)
- Remove `na_color` from `render_images()` (no use case for images)
- Remove `**kwargs` from `render_images()` (broken: crashed or was silently dropped)
- Remove dead `transfunc` from `LabelsRenderParams` (stored but never read)
- Remove vestigial `LabelsRenderParams.outline` field
- Remove vestigial `ShapesRenderParams.contour_px` field

New behavior:
- Warn when user requests `colorbar=True` for 5+ channel images

Related: #463, #582

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/_palette.py

@@ -594,8 +594,9 @@ def make_palette_from_data(
     element
         Name of a shapes or points element in *sdata*.
     color
-        Column name containing categorical labels (in the element itself
-        for points, or in the linked table for shapes/labels).
+        Column name containing categorical labels.  The column is first
+        looked up directly on the element (both for shapes and points);
+        if not found there, it falls back to the linked AnnData table.
     palette
         Source colours.  Accepts the same values as
         :func:`make_palette` (*None*, a list, a named palette, or a