Update Limitations to match current spec

[jamesnw]

Description

1. Updates the Limitations section in README against the spec as of March. https://www.w3.org/TR/css-anchor-position-1
2. Adds Status note to Demo, and deprecation notice on @position-fallback example.

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[jamesnw]

There were also some additions to the spec that may impact the polyfill logic in some way.

1. New section on "anchor relevance".
2. In creating an anchor, new specification that Pseudo elements have same implicit anchor as originating element unless otherwise specified.
3. New requirements for acceptable anchor points:
   1. el is either an element, or a [pseudo-element (https://www.w3.org/TR/selectors-4/#pseudo-element) that acts like one.
   2. el is not in the skipped contents of another element.
4. Anchor function validity- adds new requirement that there be a target element.

I also chose to mirror what the Container Query polyfill did, and specify that we will not polyfill the JS APIs.