Clarify what should happen when a container timing root has the ignore attribute

Is your feature request related to a problem? Please describe.
Some times, how nesting and containertiming-ignore works is confusing. Ignore is expected to block propagation upwards, but it is not working like that.

Describe the solution you'd like
Make it explicit in the specification what happens when an element has both containertiming and containertiming-ignore attributes.

To make the specification more useful, the proposal is that in this case, the container timing root should get events from descendants and itself, as expected, but it would not propagate upwards to any other container timing root.

This is useful for allowing to have parts of the DOM where their paints are independent from ancestors, as dialogs or overlays. Setting a container timing root to the top node allows to measure its paints, and making it ignore, allows to avoid sending paint information to other parts.

Describe alternatives you've considered
This is in the context of https://github.com/bloomberg/container-timing/blob/main/docs/container-definitions.md where we see different ways to delcare the tree. The analysis of alternatives are there.

This makes the attributes option API easier to work with, as it allows to isolate parts of the tree even while getting container timing information from that same tree.

As initially we are moving to use attributes, we need to ensure those cases are covered.

Start with docs/container-definitions.md and trace the attribute-tree alternatives described there. Clarify in the specification that an element with both containertiming and containertiming-ignore receives events from itself and descendants but does not propagate them to ancestor timing roots; ensure the documented nesting cases reflect that behavior.