Document frame rows in the asset panel (#430)

Compositions.md

@@ -138,6 +138,9 @@ the clips as narrower bars above them. The header bar has **+ Track**, **Play**
   the same sprite as its whole video, so a composition keeps one sprite per video, and the clip is
   the Texture of the item's own object. The clip rows sit under their video in the asset panel
   (see [Concepts](Concepts)). A clip can also be chosen afterwards, on the Texture row.
+- **Dragging a frame of a texture or of pixel art onto a track** (#430) works the same way: the
+  item wears the sprite of the whole picture, the frame is the Texture of its object, and it lasts
+  3 s. The frame rows sit under their texture or pixel art in the asset panel.
 - **Dragging a sound, a clip of one, a pattern or a song onto a track** (#418) makes an audio item
   there, as long as one pass of it, picked; a song's first pass ends where its loop does. A clip of
   a sound comes from its row under the sound in the asset panel (#425). A sound has to be loaded

Concepts.md

@@ -254,6 +254,13 @@ under it, folded when the project opens. A clip row is dragged like an asset, on
 song or a composition. A click on it shows the clip in the inspector without moving a tab, a
 double-click opens its sound or video with the clip picked. Folding the clips keeps a clip picked
 in its tab.
+**A texture with frames**, and **pixel art with more than one frame**, unfold their frames the same
+way (#430), the frames of pixel art named *Frame 1*, *Frame 2* ... as in the Texture picker. A
+frame row is dragged onto a track of a composition. A click on a texture's frame shows it in the
+inspector, a double-click opens the texture with the frame picked. A frame of pixel art has nothing
+of its own to show, so a click selects the pixel art and its tab, if open, paints on that frame; a
+double-click opens the tab on it. The label of a Texture row naming a frame links the frame itself:
+a click lights up its row, a double-click opens it.
 A click on a row only
 selects (the inspector shows it, no tab moves); a **double-click** opens the asset as a document
 in a tab of its own (see **The tab row**) — a behavior, state machine, shader, particles,