docs: give the full-section as_points panel full colour coverage

At the default marker size the datashader spread left white gaps, so the
whole-section panel looked grainy. Draw it with datashader and size=12 so
each cell spreads into continuous coverage, matching the overview look,
and note in the prose that size controls the spread on that backend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: timtreis