Clarify contexts in which a figcaption can be used

[Alohci]

For the figcaption element in the current HTML 5.1 draft it says:

Contexts in which this element can be used:
As a descendant of a figure element.

Elsewhere though the content model of other elements only permits a figcaption as a child of a figure element, and the semantics of figcaption say that

The figcaption element represents a caption or legend for the rest of the contents of the figcaption element’s parent figure element, if any.

So I think that either the contexts in which the figcaption element can be used to say

"As a child of a figure element."

or examples need to give explaining when it is appropriate for the element to be a non-child descendant, and whether the semantics should say

The figcaption element represents a caption or legend for the rest of the contents of the figcaption element’s nearest ancestor figure element, if any.

[chaals]

This looks like some cleanup required after fixing #177 which relaxed the restriction on where figcaption could appear.

Thanks for picking it up.