Align visualize_walks() docs and comments with actual output column names and .pluck behavior

[spsanderson]

Problem

The documentation for the visualize_walks() function refers to columns like cum_sum, cum_prod, etc., but the generators actually output columns suffixed with the dimension—such as cum_sum_y, cum_sum_x, and so on. The .pluck parameter documentation also suggests values that do not always match available column names in multi-dimensional runs, causing confusion.

Solution

• Update roxygen/documentation so parameter descriptions and return-value docs match the actual output of the generator functions (e.g., mention cum_sum_y, etc.).
• Clarify in the docs how .pluck works (index-based or column name, and supported values for 1D/2D/3D output).
• Add an example or clarify existing examples to reflect current, supported usage.

Acceptance Criteria

• Roxygen and reference documentation match the actual (suffixed) column names seen in real outputs.
• .pluck documentation is clear and correct.
• Examples in help and README work as shown.