ASoC: Update snd_soc_dai_set_tdm_slot() documentation

There have been some conflicting interpretations of how
snd_soc_dai_set_tdm_slot() is supposed to work. This patch updates the
documentation to be more specific on the exact semantics to avoid such
problems in the future.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>