cleanup: More comments in timer.h

Indicate when usleep() and udelay() may be called.

No code changes, just comments.

BUG=none
BRANCH=none
TEST=Build any platform.  Heck, it's just comments.

Change-Id: I0182c153c29965b25d5294d838c1406c30115099
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/177452
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

include/timer.h

@@ -60,6 +60,9 @@ int timestamp_expired(timestamp_t deadline, const timestamp_t *now);
 /**
  * Busy-wait.
  *
+ * This may be called with interrupts disabled, at any time after timer_init()
+ * has been called.
+ *
  * Note that calling this with us>1000 may impact system performance; use
  * usleep() for longer delays.
  *
@@ -74,12 +77,16 @@ void udelay(unsigned us);
  * perhaps longer, if a higher-priority task is running when the delay
  * expires).
  *
+ * This may only be called from a task function, with interrupts enabled.
+ *
  * @param us		Number of microseconds to sleep.
  */
 void usleep(unsigned us);
 
 /**
- * Sleep for milliseconds
+ * Sleep for milliseconds.
+ *
+ * Otherwise the same as usleep().
  *
  * @param ms		Number of milliseconds to sleep.
  */
@@ -91,6 +98,8 @@ static inline void msleep(unsigned ms)
 /**
  * Sleep for seconds
  *
+ * Otherwise the same as usleep().
+ *
  * @param sec		Number of seconds to sleep.
  */
 static inline void sleep(unsigned sec)