Fixing uncommented classes for the JOSS paper.

See details at:
https://github.com/openjournals/joss-reviews/issues/251#issuecomment-300128786

src/Grafoscopio/GrafoscopioButtonModel.class.st

@@ -1,3 +1,9 @@
+"
+I'm just a prototype of how morphic buttons can be embedded
+inside a Grafoscopio notebook UI via Spec.
+I'm not used in the actual UI, but I'm more a remainder of directions
+to explore.
+"
 Class {
 	#name : #GrafoscopioButtonModel,
 	#superclass : #ComposableModel,

src/Grafoscopio/GrafoscopioCodeModel.class.st

@@ -1,3 +1,7 @@
+"
+I represent a Pharo code node inside a Grafoscopio notebook as a 
+embedded interactive Playground.
+"
 Class {
 	#name : #GrafoscopioCodeModel,
 	#superclass : #ComposableModel,

src/Grafoscopio/GrafoscopioTextModel.class.st

@@ -1,3 +1,7 @@
+"
+I represent a text node inside a Grafoscopio notebook.
+Usually my content is markdown text.
+"
 Class {
 	#name : #GrafoscopioTextModel,
 	#superclass : #ComposableModel,