Clarify `contenteditable="false"` elements which is associated with an `EditContext`

https://w3c.github.io/edit-context/#edit-context-differences

Each child node of the EditContext editing host becomes editable, unless that node has a contenteditable attribute set to "false".

I guess that if the element whose contenteditable is "false" has an EditContext, it should be editable because if only the element has focus, it should be editable.

So in my understanding an element can be editable if:

• designMode is set to on
• contenteditable is set to true
• an EditContext is associated
• an ancestor is editable and not in designMode and not associated with an EditContext and contenteditable is set to "false"

I.e., an element in an editing host can be non-editable if and only if:

• not in designMode
• not associated with an EditContext
• contenteditable is set to "false"
• or a descendant of the non-editable element and is not an editing host

Cc: @dandclark

Read the EditContext differences section and the linked HTML definitions for contenteditable and editable. Compare the quoted rule with the proposed cases for elements associated with an EditContext, then clarify the normative behavior in the specification. Done means the relationship between contenteditable="false", focus, editing hosts, and EditContext is unambiguous.