ASoC: dt-bindings: audio-graph-card: Expand 'widgets' documentation

Document the encoding of 'widgets' property to avoid confusion.

Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230606175951.215740-1-marex@denx.de


Signed-off-by: Mark Brown <broonie@kernel.org>