doc: add notes that timer jobs should be side-effect-free (#682)

... other than submitting the self-call, of course

src/Timer.mo

@@ -21,6 +21,9 @@ module {
   /// Installs a one-off timer that upon expiration after given duration `d`
   /// executes the future `job()`.
   ///
+  /// Note: Since `job` can be called more than once (under rare circumstances),
+  ///       it should not possess side-effects.
+  ///
   /// ```motoko no-repl
   /// let now = Time.now();
   /// let thirtyMinutes = 1_000_000_000 * 60 * 30;
@@ -37,6 +40,8 @@ module {
   /// executes the future `job()` and reinserts itself for another expiration.
   ///
   /// Note: A duration of 0 will only expire once.
+  /// Note: Since `job` can be called more than once (under rare circumstances),
+  ///       it should not possess side-effects.
   ///
   /// ```motoko no-repl
   /// func checkAndWaterPlants() : async () {