aom_image.h: add lifetime note for img_data

The wrapped storage must exist for the duration of the aom_image_t
allocation.

Bug: aomedia:363806063
Change-Id: Ic6b79a56b6c07776222d1767490d873d7408ced0

aom/aom_image.h

diff --git a/aom/aom_image.h b/aom/aom_image.h
index 1b790d5..248b5b6 100644
--- a/aom/aom_image.h
+++ b/aom/aom_image.h
@@ -278,7 +278,9 @@
  *                         (2^27).
  * \param[in]    align     Alignment, in bytes, of each row in the image
  *                         (stride). Must not exceed 65536.
- * \param[in]    img_data  Storage to use for the image
+ * \param[in]    img_data  Storage to use for the image. The storage must
+ *                         outlive the returned image descriptor; it can be
+ *                         disposed of after calling aom_img_free().
  *
  * \return Returns a pointer to the initialized image descriptor. If the img
  *         parameter is non-null, the value of the img parameter will be