[css-pseudo] text decorations on highlight pseudos (closes #6022)

[delan]

This patch makes some further edits for #6022, clarifying that our rules for painting text decorations apply to both line decorations and emphasis marks, and that in effect, decorations will never be obscured by highlight backgrounds.

[fantasai]

Looks good, just one thing:

• Don't add the word “Other”, it's “{Text} and {Text Decorations}” not “{Text which is a type of Text Decoration} and {Other Types of Text Decorations}”!
• Squash+Rebase when you merge. (See drop-down next to GH's Merge button.)

[delan]

Thanks @fantasai, done in 99cfe20. Yeah, that heading would be better clarified by something like “Text, Line Decorations, and Emphasis Marks”, but the body makes the same clarification anyway so it’s not really necessary.

[delan]

@fantasai looks like I don’t have write access, could you please merge?

[frivoal]

I'm not @fantasai, but I do have merge access :) Merging.