add a note about memory leaks to Drawer and Sprite docs

drawer.go

@@ -15,6 +15,11 @@ package pixel
 //
 // Whenever you change the Triangles, call Dirty to notify Drawer that Triangles changed. You don't
 // need to notify Drawer about a change of the Picture.
+//
+// Note, that Drawer caches the results of MakePicture from Targets it's drawn to for each Picture
+// it's set to. What it means is that using a Drawer with an unbounded number of Pictures leads to a
+// memory leak, since Drawer caches them and never forgets. In such a situation, create a new Drawer
+// for each Picture.
 type Drawer struct {
 	Triangles Triangles
 	Picture   Picture

sprite.go

@@ -11,6 +11,11 @@ import "image/color"
 //
 // To achieve different anchoring, transformations and color masking, use SetMatrix and SetColorMask
 // methods.
+//
+// Note, that Sprite caches the results of MakePicture from Targets it's drawn to for each Picture
+// it's set to. What it means is that using a Sprite with an unbounded number of Pictures leads to a
+// memory leak, since Sprite caches them and never forgets. In such a situation, create a new Sprite
+// for each Picture.
 type Sprite struct {
 	tri   *TrianglesData
 	frame Rect