clk: update clk API documentation to clarify clk_round_rate()

The idea is that rate = clk_round_rate(clk, r) is equivalent to:

	clk_set_rate(clk, r);
	rate = clk_get_rate(clk);

except that clk_round_rate() does not change the hardware in any way.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

include/linux/clk.h

@@ -306,6 +306,20 @@ void devm_clk_put(struct device *dev, struct clk *clk);
  * @clk: clock source
  * @rate: desired clock rate in Hz
  *
+ * This answers the question "if I were to pass @rate to clk_set_rate(),
+ * what clock rate would I end up with?" without changing the hardware
+ * in any way.  In other words:
+ *
+ *   rate = clk_round_rate(clk, r);
+ *
+ * and:
+ *
+ *   clk_set_rate(clk, r);
+ *   rate = clk_get_rate(clk);
+ *
+ * are equivalent except the former does not modify the clock hardware
+ * in any way.
+ *
  * Returns rounded clock rate in Hz, or negative errno.
  */
 long clk_round_rate(struct clk *clk, unsigned long rate);