Update comment for avifDecoderItem::mergedExtents

The mergedExtents field of the avifDecoderItem struct is now always
used. However, it may or may not own the buffer it points to, depending
on the value of the ownsMergedExtents field.

src/read.c

diff --git a/src/read.c b/src/read.c
index f46a9c8..984e10f 100644
--- a/src/read.c
+++ b/src/read.c
@@ -222,8 +222,10 @@
     avifContentType contentType;
     avifPropertyArray properties;
     avifExtentArray extents;       // All extent offsets/sizes
-    avifRWData mergedExtents;      // if set, is a single contiguous block of this item's extents (unused when extents.count == 1)
-    avifBool ownsMergedExtents;    // if true, mergedExtents must be freed when this item is destroyed
+    avifRWData mergedExtents;      // A single contiguous block of this item's extents
+    avifBool ownsMergedExtents;    // If true, mergedExtents must be freed when this item is destroyed.
+                                   // If false, mergedExtents is used as an avifROData and points to a
+                                   // buffer it doesn't own.
     avifBool partialMergedExtents; // If true, mergedExtents doesn't have all of the item data yet
     uint32_t thumbnailForID;       // if non-zero, this item is a thumbnail for Item #{thumbnailForID}
     uint32_t auxForID;             // if non-zero, this item is an auxC plane for Item #{auxForID}