ResizeObserver bindings are not documented

[ebeloded]

Describe the problem

ResizeObserver bindings (bind:contentRect|contentBoxSize|borderBoxSize|devicePixelContentBoxSize) implemented in #8022 are not documented anywhere.

Describe the proposed solution

Add the missing info to Element Directives section of the docs.

Importance

would make my life easier

[Prinzhorn]

#8630
#10269

[ebeloded]

Sorry. Duplicate.