Docs: info about how animated images are loaded

Author: lovell

docs/api-constructor.md

@@ -17,6 +17,10 @@ Non-critical problems encountered during processing are emitted as `warning` eve
 
 Implements the [stream.Duplex](http://nodejs.org/api/stream.html#stream_class_stream_duplex) class.
 
+When loading more than one page/frame of an animated image,
+these are combined as a vertically-stacked "toilet roll" image
+where the overall height is the `pageHeight` multiplied by the number of `pages`.
+
 **Throws**:
 
 - <code>Error</code> Invalid parameters

lib/constructor.js

@@ -22,6 +22,10 @@ const debuglog = util.debuglog('sharp');
  *
  * Implements the [stream.Duplex](http://nodejs.org/api/stream.html#stream_class_stream_duplex) class.
  *
+ * When loading more than one page/frame of an animated image,
+ * these are combined as a vertically-stacked "toilet roll" image
+ * where the overall height is the `pageHeight` multiplied by the number of `pages`.
+ *
  * @constructs Sharp
  *
  * @emits Sharp#info