Update unsupported features page to mention emptyElementParsePolicy

Until DAFFODIL-2496 is fixed, our implementation of emptyElementParsePolicy is as an extension in the dfdlx namespace.

The unsupported features list should mention this.

See also DAFFODIL-2179 which is a doc page for all the extensions,  which should mention emptyElementParsePolicy until DAFFODIL-2496 is fixed.