mmc: core: document mmc_hw_reset()

Add documentation for mmc_hw_reset to make sure the intended use case is
clear.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20200918215446.65654-1-wsa+renesas@sang-engineering.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/core.c

@@ -2063,6 +2063,16 @@ static void mmc_hw_reset_for_init(struct mmc_host *host)
 	host->ops->hw_reset(host);
 }
 
+/**
+ * mmc_hw_reset - reset the card in hardware
+ * @host: MMC host to which the card is attached
+ *
+ * Hard reset the card. This function is only for upper layers, like the
+ * block layer or card drivers. You cannot use it in host drivers (struct
+ * mmc_card might be gone then).
+ *
+ * Return: 0 on success, -errno on failure
+ */
 int mmc_hw_reset(struct mmc_host *host)
 {
 	int ret;