DOC: Add note about simplification of to_polygons

[greglucas]

PR summary

During a review in Cartopy, it was noted that it was non-obvious that Path.to_polygons would also simplify the path. https://github.com/SciTools/cartopy/pull/2362#issuecomment-2179511074 This adds an explicit note that it will simplify the path according to the should_simplify attribute.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• [n/a] new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• [x] Documentation complies with general and docstring guidelines