rtc: clarify the RTC offset correction

The RTC offset correction documentation is not very clear about the
exact relationship between "offset" and the effect it has on the RTC.
Supplement the documentation with an equation giving the relationship.

Signed-off-by: Russell King <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/interface.c

···
  * to compensate for differences in the actual clock rate due to temperature,
  * the crystal, capacitor, etc.
  *
+ * The adjustment applied is as follows:
+ *   t = t0 * (1 + offset * 1e-9)
+ * where t0 is the measured length of 1 RTC second with offset = 0
+ *
  * Kernel interface to adjust an rtc clock offset.
  * Return 0 on success, or a negative number on error.
  * If the rtc offset is not setable (or not implemented), return -EINVAL