Fix user migration guide: widgetsnbextension no longer requires notebook.

Author: jasongrout

docs/source/migration_guides.md

@@ -1,7 +1,7 @@
 Migrating custom widget libraries
 =================================
 
-These are migration guides aimed specifically at developers of third-party
+These are migration guides specifically for developers of third-party
 widgets.
 
 Migrating from 7.x to 8.0

docs/source/user_migration_guides.md

@@ -1,7 +1,7 @@
 Migrating user code
 ===================
 
-These are migration guides aimed specifically at user of ipywidgets.
+These are migration guides specifically for ipywidgets users.
 
 Migrating from 7.x to 8.0
 -------------------------
@@ -85,6 +85,7 @@ attribute `data-jupyter-widgets-cdn` on the HTML manager script tag. See
 
 #### widgetsnbextension
 
-The `widgetsnbextension` package is no longer a dependency of `ipywidgets`. Consequently,
-neither is the `notebook` package. If you need to keep `notebook` and widget support for it
-you will need to ensure they are explicitly stated in any environment bootstrapping.
+The `notebook` package is no longer a dependency of the `widgetsnbextension`
+package (therefore `notebook` is no longer a dependency of `ipywidgets`). If you
+need to install `notebook` with `ipywidgets`, you will need to install
+`notebook` explicitly.