Auto merge of #14100 - servo:display_list_glossary, r=emilio

Define "display list"

Fix a TODO in the Glossary.

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- [ ] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix no issue; there wasn't one filed AFAIK
- [X] These changes do not require tests because reasons

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docs/glossary.md

@@ -16,7 +16,7 @@ The thread that controls a collection of related web content. This could be thou
 
 ### Display list ###
 
-TODO
+A list of concrete rendering instructions. The display list is post-layout, so all items have stacking-context-relative pixel positions, and z-index has already been applied, so items later in the display list will always be on top of items earlier in it.
 
 ### Layout thread ###