resolves #325

This PR revises the mappings for `figure` and `figcaption` to remove the "labelled by" relations, changing them to "details for".
The accessible name computation for `figure` removes `figcaption`, a note is added to further reference this change.

TODO: 
- [ ] need to check on updates to UIA mapping, if any.
- [ ] do we want `figcaption` to map to `caption`? 
- [ ] regardless of above, [`caption`](https://w3c.github.io/aria/#caption) will need to be updated in regards to its referencing `figure` and `figcaption` relationships
- [ ] [`aria-details`](https://w3c.github.io/aria/#aria-details) definition should be updated to reference this change

cc @cookiecrook to make sure i got this right.  thank you

verified the UIA mappings with help from @benbeaudry 
made some of the wording consistent between mappings
added in a SHOULD to indicate that if a figure is explicitly provided an accessible name from its figcaption, then do not expose the details relationship, as that'd add redundancy.

saying it doesn't know what accname is... so, replacing the instances of the links which may be causing the issue?

see https://bugs.chromium.org/p/chromium/issues/detail?id=1426613#c4

aria-details with an empty string could be useful to authors if there was a specific need to _not_ expose a child figcaption - e.g., if the elements need to have remapped roles for whatever reason.

cc @cookiecrook