doc: document oiiotool --layersplit command

Signed-off-by: Loïc Vital <[email protected]>

Author: mugulmd

src/doc/oiiotool.rst

@@ -2193,6 +2193,16 @@ current top image.
     image comprised of the subimages of all original images appended
     together.
 
+.. option:: --layersplit
+
+    Remove the top image from the stack, split it into its consituent
+    channel-name-based layers, and push them all onto the stack
+    (first to last).
+
+    By "layer" we mean a subset of the initial channels which, when named
+    using the convention "LAYERNAME.channelname", all share the same layer
+    name.
+
 .. option:: --ch <channellist>
 
     Replaces the top image with a new image whose channels have been