[libadwaita] animation: Document ADW_DURATION_INFINITE

[Alexander Mikhaylenko <alexm src gnome org>]

commit 0da75ede43e08d1914c920be82a2140a89cd0798
Author: Maximiliano Sandoval R <msandova gnome org>
Date:   Tue Dec 7 12:34:20 2021 +0100

    animation: Document ADW_DURATION_INFINITE

 src/adw-animation.h        | 8 ++++++++
 src/adw-spring-animation.c | 2 ++
 2 files changed, 10 insertions(+)
---
diff --git a/src/adw-animation.h b/src/adw-animation.h
index ff075e07..33fc8df3 100644
--- a/src/adw-animation.h
+++ b/src/adw-animation.h
@@ -20,6 +20,14 @@
 
 G_BEGIN_DECLS
 
+/**
+ * ADW_DURATION_INFINITE:
+ *
+ * Indicates an [class@Adw.Animation] with an infinite duration.
+ *
+ * This value is mostly used internally.
+ */
+
 #define ADW_DURATION_INFINITE (0xffffffffU)
 
 #define ADW_TYPE_ANIMATION (adw_animation_get_type())
diff --git a/src/adw-spring-animation.c b/src/adw-spring-animation.c
index b9e48070..a849c5fd 100644
--- a/src/adw-spring-animation.c
+++ b/src/adw-spring-animation.c
@@ -512,6 +512,8 @@ adw_spring_animation_class_init (AdwSpringAnimationClass *klass)
    *
    * Estimated duration of the animation in milliseconds.
    *
+   * Can be [const@Adw.DURATION_INFINITE] if the spring damping is set to 0.
+   *
    * Since: 1.0
    */
   props[PROP_ESTIMATED_DURATION] =