Clarify container queries always use content box (fix #40306) (#40450)

* Clarify container queries always use content box (fix #40306)

* Apply suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Added the changes in one place

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Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: 3 people

files/en-us/web/css/@container/index.md

@@ -127,7 +127,7 @@ The `<container-condition>` queries include [size](#size_container_descriptors)
 
 #### Size container descriptors
 
-The `<container-condition>` can include one or more boolean size queries, each within a set of parentheses. A size query includes a size descriptor, a value, and — depending on the descriptor — a comparison operator. The syntax for including multiple conditions is the same as for [`@media`](/en-US/docs/Web/CSS/@media) size feature queries.
+The `<container-condition>` can include one or more boolean size queries, each within a set of parentheses. A size query includes a size descriptor, a value, and — depending on the descriptor — a comparison operator. The queries always measures the [content box](/en-US/docs/Web/CSS/box-edge#content-box) as the comparison. The syntax for including multiple conditions is the same as for [`@media`](/en-US/docs/Web/CSS/@media) size feature queries.
 
 ```css
 @container (min-width: 400px) {