docs: clarify markevery float spacing - #31664

PR summary

Closes #27842.

Clarify the Line2D.set_markevery docstring for float markevery values. The updated wording explains that the ideal spacing is measured along the drawn line and then coerced to the nearest data point, so jagged lines may not appear evenly spaced along the x- or y-axis.

Validation:

• python -m py_compile lib\matplotlib\lines.py
• git diff --check
• Parsed lib/matplotlib/lines.py with ast to confirm the updated Line2D.set_markevery docstring contains the new wording.

AI Disclosure

I used Codex locally to help identify the issue, draft the wording, and run validation. I reviewed the final one-file diff before opening this PR.

PR checklist

• "closes [DOC]: clarify that markevery[float] considers path length along drawn line #27842" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested: this is a docstring-only change, validated with py_compile, git diff --check, and an AST docstring check
• [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
• Documentation complies with general and docstring guidelines