Clarify comment (hopefully)

Author: jdemeyer

src/bin/sage-env

@@ -371,14 +371,20 @@ if [ "$SAGE_STARTUP_FILE" = "" ]; then
 fi
 
 if [ -z "$IPYTHONDIR" ]; then
-    # Use 5.0.0 in the directory name for all IPython versions
-    # compatible with that version.
+    # We hardcode a version number in the directory name. The idea is
+    # that we keep using the same version number as long as that is
+    # possible. Only when some future IPython version really requires
+    # a new structure for the $IPYTHONDIR should this version number be
+    # changed to the new IPython version.
     export IPYTHONDIR="$DOT_SAGE/ipython-5.0.0"
 fi
 
 if [ -z "$JUPYTER_CONFIG_DIR" ]; then
-    # Use 4.1 in the directory name for all jupyter_core versions
-    # compatible with that version.
+    # We hardcode a version number in the directory name. The idea is
+    # that we keep using the same version number as long as that is
+    # possible. Only when some future Jupyter version really requires
+    # a new structure for the $JUPYTER_CONFIG_DIR should this version
+    # number be changed to the new jupyter_core version.
     export JUPYTER_CONFIG_DIR="$DOT_SAGE/jupyter-4.1"
 fi