document some modules that are referenced in other documentation

collects/mrlib/scribblings/matrix-snip.scrbl

@@ -0,0 +1,16 @@
+#lang scribble/doc
+@(require "common.ss"
+          (for-label mrlib/matrix-snip
+                     (only-in mrlib/cache-image-snip
+                              cache-image-snip%)))
+
+@title{Matrix Snip}
+
+@defmodule[mrlib/matrix-snip]
+
+The @racket[mrlib/matrix-snip] library implements a matrix value that displays
+in 2-D graphical form.
+
+@defclass[visible-matrix% cache-image-snip% ()]{
+
+A 2-D graphical matrix.}

collects/mrlib/scribblings/mrlib.scrbl

@@ -19,6 +19,7 @@
 @include-section["plot.scrbl"]
 @include-section["switchable-button.scrbl"]
 @include-section["image-core.scrbl"]
+@include-section["matrix-snip.scrbl"]
 @include-section["tex-table.scrbl"]
 
 @section{Acknowledgments}

collects/scribblings/scheme/scheme.scrbl

@@ -356,6 +356,12 @@ Both @schememodname[scheme/gui/base] and
 @schememodname[racket/gui/base] depend on @schememodname[mred], so it
 is attached by all variants of @racket[make-gui-empty-namespace].
 
+@defmodule*/no-declare[(mred/mred)]
+
+The @schememodname[mred] library actually just re-exports
+@schememodname[mred/mred], which is an even older name for the
+library.
+
 @; ----------------------------------------
 
 @include-section["compat.scrbl"]