docs: do not use Instant::now in #[init]

examples/schedule.rs

@@ -20,6 +20,7 @@ const APP: () = {
         cx.core.DWT.enable_cycle_counter();
 
         // semantically, the monotonic timer is frozen at time "zero" during `init`
+        // NOTE do *not* call `Instant::now` in this context; it will return a nonsense value
         let now = cx.start; // the start time of the system
 
         hprintln!("init @ {:?}", now).unwrap();

src/cyccnt.rs

@@ -30,6 +30,10 @@ pub struct Instant {
 
 impl Instant {
     /// Returns an instant corresponding to "now"
+    ///
+    /// *HEADS UP* this function can, and will, return nonsensical values if called within `init`.
+    /// Only use it in `idle` and tasks. In `init`, use the `init::Context.start` field, or the
+    /// `CYCCNT::zero` function, instead of this function
     pub fn now() -> Self {
         Instant {
             inner: DWT::get_cycle_count() as i32,