Fixes for migration docs

Author: jasongrout

docs/source/migration_guides.md

@@ -117,7 +117,7 @@ The ``DOMWidgetView.processPhosphorMessage`` method has been renamed ``DOMWidget
 + }
 ```
 
-I you're dropping ipywidgets 7.x support, you can simply rename the `processPhosphorMessage` method into `processLuminoMessage`.
+If you're dropping ipywidgets 7.x support, you can simply rename the `processPhosphorMessage` method into `processLuminoMessage`.
 
 #### Widget manager import
 
@@ -128,7 +128,7 @@ As mentioned before, if you depend on the ``ManagerBase`` class, you will **eith
 + import { ManagerBase } from '@jupyter-widgets/base-manager';
 ```
 
-**or**, siwtch to using the new `IWidgetManager` interface in the `base` package:
+**or**, switch to using the new `IWidgetManager` interface in the `base` package:
 
 ```diff
 - import { ManagerBase } from '@jupyter-widgets/base';
@@ -153,7 +153,7 @@ export async function myDeserializer(
 
 #### Backbone extend
 
-The version of backbone that ipywidgets depend on has changed from 1.2.3 to 1.4.0. If you were extending the base widget model with `var CustomWidgetModel = Widget.extend({ ... });` you will need to update the class definition using the ES6 notation:
+The version of [Backbone.js](https://backbonejs.org/) that ipywidgets depends on has changed from 1.2.3 to 1.4.0. If you were extending the base widget model with `var CustomWidgetModel = Widget.extend({ ... });` you will need to update the class definition using the ES6 notation:
 
 ```diff
 - var CustomWidgetModel = Widget.extend({
@@ -166,6 +166,18 @@ The version of backbone that ipywidgets depend on has changed from 1.2.3 to 1.4.
 
 Note: If you were relying on setting certain instance attributes via the `extend` method, you might now need override the `preinitialize` method in order for their values to be set in time.
 
+#### Custom tag names
+
+If you were changing the base HTML tag by defining the `tagName` property, this can now be done in the `preinitialize` method (see https://github.com/jupyter-widgets/ipywidgets/commit/a342e0dbc7c779bb668e5a21c097d7cec9a6ac44 for example changes in core widgets):
+
+```diff
+- get tagName() {
+-   return 'button';
+- }
++ preinitialize() {
++   this.tagName = 'button';
++ }
+```
 
 Migrating from 6.0 to 7.0
 -------------------------