Improve understandability of Figure in "Rules of ARIA attribute usage by HTML element" table

[giacomo-petri]

• URL: https://www.w3.org/TR/html-aria/#el-figure

• Issue description:
  The existing content in the "ARIA role, state, and property allowances" cell for the Figure is as follows:

  If the figure has no figcaption descendant:
  Any role, though figure is NOT RECOMMENDED.

  If the figure has a figcaption descendant:
  DPub Role: doc-example.

  Otherwise, figure is allowed, but NOT RECOMMENDED.

  Global aria-* attributes and any aria-* attributes applicable to the allowed roles.

  The if-if-else structure is perplexing because the "otherwise" scenario may never be applicable, given that the second "if" statement is the opposite of the first one and covers the entirety of the options (if has no figcaption descendant, if has figcaption descendant, otherwise).

  Additionally, the "Otherwise" condition diverges from the typical instances found in the document, where it generally advises "No role other than ***, which is NOT RECOMMENDED.".

  I am submitting a pull request that introduces a suggested resolution to address these concerns.