Explain why col backgrounds can be hidden by cell backgrounds

[quepasaquepasa]

Fixes #44816

The reporter followed the article's own instructions — copy blank-template.html + minimal-table.css, then try the <colgroup>/<col> shading — and the column colors never appeared.

That's expected once you know the painting order: per CSS tables, backgrounds are painted table → column group → column → row group → row → cell, so a cell's own background covers the column's. minimal-table.css sets one on th and on alternating rows:

th { background-color: rgb(235,235,235); }
tr:nth-child(even) td { background-color: rgb(250,250,250); }
tr:nth-child(odd) td { background-color: rgb(245,245,245); }

so every cell in the table already has a background and nothing from <col> shows through.

The live sample on the page is unaffected — its hidden CSS only sets border/padding on td, th, no background — which is why the example renders correctly on MDN while the reader's copy does not.

Added a note explaining the painting order and pointing out which rules to remove from the stylesheet copy. I kept it to a note rather than changing minimal-table.css in the learning-area repo, since those backgrounds are deliberate elsewhere in the module.