Update docs on Scheduler.cue edge case handling

doc/Type/Scheduler.pod6

@@ -51,10 +51,11 @@ C<$times> tells the scheduler how many times to run the code.
 C<&catch> is called with the L<Exception|/type/Exception> as its sole argument
 if C<&code> dies.
 
-If C<$at>, C<$in>, or C<$every> are C<Inf>, C<&code> will never be run. If any
-of them are C<-Inf>, C<&code> will be run immediately. If any of them are
-C<NaN>, an C<X::AdHoc> exception will be thrown with the message "Cannot set
-NaN as a number of seconds".
+If C<$at> or C<$in> are C<Inf>, C<&code> will never be run; if C<$every> is
+C<Inf>, C<&code> will only be run once. If any of the three are C<-Inf>,
+C<&code> will be run immediately. If any of the three are C<NaN>, an
+L<X::Scheduler::CueInNaNSeconds|/type/X/Scheduler/CueInNanSeconds> exception
+will be thrown. This only applies to versions 2019.04 and later.
 
 One should call the C<cancel> method on the returned C<Cancellation> object
 to cancel the (possibly repeated) cueing of the code.

doc/Type/X/Scheduler/CueInNaNSeconds.pod6

@@ -0,0 +1,23 @@
+=begin pod
+
+=TITLE class X::Scheduler::CueInNaNSeconds
+
+=SUBTITLE Error caused by passing NaN to Scheduler.cue as :at, :in, or :every
+
+    class X::Scheduler::CueInNaNSeconds is Exception { }
+
+In versions 2019.04 and later, when calling C<ThreadPoolScheduler.cue> or
+C<CurrentThreadScheduler.cue> with C<:at>, C<:in>, or C<:every> as C<NaN>, this
+exception gets thrown. For example, the following code:
+
+=for code :skip-test
+my Cancellation $c = $*SCHEDULER.cue({
+    say 'This will never output :(';
+}, at => NaN);
+
+Throws with:
+
+=for code :lang<text>
+Cannot pass NaN as a number of seconds
+
+=end pod