Update `TravelEstimate` and `NavigationTemplate` javadoc on when an unknown time is not allowed

Relnote: Updated javadoc on when setting a `TravelEstimate`'s remaining time to an unknown time is allowed.
Test: n/a
Bug: 183632456
Change-Id: I9961081b996822fae8f8763abe578cb622a3d23d
(cherry picked from commit b699061d27012a15895eaa257bda62fff77bf9fe)

diff --git a/car/app/app/src/main/java/androidx/car/app/navigation/model/NavigationTemplate.java b/car/app/app/src/main/java/androidx/car/app/navigation/model/NavigationTemplate.java
index 47ff35a..a7a2d10 100644
--- a/car/app/app/src/main/java/androidx/car/app/navigation/model/NavigationTemplate.java
+++ b/car/app/app/src/main/java/androidx/car/app/navigation/model/NavigationTemplate.java

@@ -208,7 +208,8 @@
          * Sets the {@link TravelEstimate} to the final destination.
          *
          * @throws IllegalArgumentException if the {@link TravelEstimate}'s remaining time is
-         *                                  less than zero
+         *                                  {@link TravelEstimate#REMAINING_TIME_UNKNOWN} or less
+         *                                  than zero
          * @throws NullPointerException     if {@code destinationTravelEstimate} is {@code null}
          */
         @NonNull

diff --git a/car/app/app/src/main/java/androidx/car/app/navigation/model/TravelEstimate.java b/car/app/app/src/main/java/androidx/car/app/navigation/model/TravelEstimate.java
index 42b162c..558bc8d 100644
--- a/car/app/app/src/main/java/androidx/car/app/navigation/model/TravelEstimate.java
+++ b/car/app/app/src/main/java/androidx/car/app/navigation/model/TravelEstimate.java

@@ -213,6 +213,10 @@
          *
          * <p>If not set, {@link #REMAINING_TIME_UNKNOWN} will be used.
          *
+         * <p>Note that {@link #REMAINING_TIME_UNKNOWN} may not be supported depending on where the
+         * {@link TravelEstimate} is used. See the documentation of where {@link TravelEstimate}
+         * is used for any restrictions that might apply.
+         *
          * @throws IllegalArgumentException if {@code remainingTimeSeconds} is a negative value
          *                                  but not {@link #REMAINING_TIME_UNKNOWN}
          */