clk: Improve documentation for devm_clk_get() and its optional variant

Make use of "Context:" and "Return:". Mention that the clk is not to be
expected to be prepared, previously only not being enabled was mentioned
which probably dates from the times when the concept of clk preparation
wasn't invented yet.

Also describe devm_clk_get_optional() fully instead of just referencing
devm_clk_get().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220520075737.758761-2-u.kleine-koenig@pengutronix.de
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

include/linux/clk.h

@@ -443,15 +443,16 @@ int __must_check devm_clk_bulk_get_all(struct device *dev,
  * @dev: device for clock "consumer"
  * @id: clock consumer ID
  *
- * Returns a struct clk corresponding to the clock producer, or
+ * Context: May sleep.
+ *
+ * Return: a struct clk corresponding to the clock producer, or
  * valid IS_ERR() condition containing errno.  The implementation
  * uses @dev and @id to determine the clock consumer, and thereby
  * the clock producer.  (IOW, @id may be identical strings, but
  * clk_get may return different clock producers depending on @dev.)
  *
- * Drivers must assume that the clock source is not enabled.
- *
- * devm_clk_get should not be called from within interrupt context.
+ * Drivers must assume that the clock source is neither prepared nor
+ * enabled.
  *
  * The clock will automatically be freed when the device is unbound
  * from the bus.
@@ -464,8 +465,20 @@ struct clk *devm_clk_get(struct device *dev, const char *id);
  * @dev: device for clock "consumer"
  * @id: clock consumer ID
  *
- * Behaves the same as devm_clk_get() except where there is no clock producer.
- * In this case, instead of returning -ENOENT, the function returns NULL.
+ * Context: May sleep.
+ *
+ * Return: a struct clk corresponding to the clock producer, or
+ * valid IS_ERR() condition containing errno.  The implementation
+ * uses @dev and @id to determine the clock consumer, and thereby
+ * the clock producer.  If no such clk is found, it returns NULL
+ * which serves as a dummy clk.  That's the only difference compared
+ * to devm_clk_get().
+ *
+ * Drivers must assume that the clock source is neither prepared nor
+ * enabled.
+ *
+ * The clock will automatically be freed when the device is unbound
+ * from the bus.
  */
 struct clk *devm_clk_get_optional(struct device *dev, const char *id);