Merge pull request #555 from lurch/patch-1

RTC doc fix
diff --git a/stmhal/rtc.c b/stmhal/rtc.c
index ec3c012..ea7aa78 100644
--- a/stmhal/rtc.c
+++ b/stmhal/rtc.c
@@ -260,7 +260,7 @@
 ///
 /// `weekday` is 1-7 for Monday through Sunday.
 ///
-/// `subseconds` is 0-59.
+/// `subseconds` counts down from 255 to 0
 mp_obj_t pyb_rtc_datetime(uint n_args, const mp_obj_t *args) {
     if (n_args == 1) {
         // get date and time