Fixes based on feedback and rename the file. (#501)

* Add ASPNET Accessibility Compat Document

* Update ASPNET Accessibility Improvement.md

* Update ASPNET Accessibility Improvement.md

@rpetrusha incorporated feedback

* ASPNET Accessibility Improvement.md

* minor fixups

* Add more details and rename the file

Documentation/compatibility/ASPNET-accessibility-improvement.md

@@ -0,0 +1,50 @@
+## ASP.NET Accessibility Improvement in .NET 4.7.1
+
+### Scope
+Minor
+
+
+### Version Introduced
+.NET 4.7.1
+
+
+### Source Analyzer Status
+NotPlanned
+
+
+### Change Description
+Starting with the .NET Framework 4.7.1, ASP.NET has improved how ASP.NET Web Controls work with accessibility technology in Visual Studio to better support ASP.NET customers.  These include the following changes:
+- Changes to implement missing UI accessibility patterns in controls, like the Add Field dialog in the Details View wizard, or the Configure ListView dialog of the ListView wizard.
+- Changes to improve the display in High Contrast mode, like the Data Pager Fields Editor.
+- Changes to improve the keyboard navigation experiences for controls, like the Fields dialog in the Edit Pager Fields wizard of the DataPager control, the Configure ObjectContext dialog, or the Configure Data Selction dialog of the Configure Data Source wizard.
+
+- [x] Quirked 
+- [ ] Build-time break 
+
+
+### Recommended Action
+__How to opt in or out of these changes__
+
+In order for the Visual Studio Designer to benefit from these changes, it must run on the .NET Framework 4.7.1 or later. The web application can benefit from these changes in either of the following ways:
+- Install Visual Studio 2017 15.3 or later, which supports the new accessibility features with the following AppContext Switch by default.
+- Opt out of the legacy accessibility behaviors by adding the __Switch.UseLegacyAccessibility__ AppContext Switch to the ```<runtime>``` section in the devenv.exe.config file and setting it to ```false```, as the following example shows.
+```
+    <?xml version="1.0" encoding="utf-8"?>
+    <configuration>
+      <runtime>
+      ...
+        <!-- AppContextSwitchOverrides value attribute is in the form of 'key1=true|false;key2=true|false  -->
+      <AppContextSwitchOverrides value="...;Switch.UseLegacyAccessibilityFeatures=false" />
+      ...
+      </runtime>
+    </configuration>
+```
+Applications that target the .NET Framework 4.7.1 or later and want to preserve the legacy accessibility behavior can opt in to the use of legacy accessibility features by explicitly setting this AppContext switch to ```true```.
+
+### Affected APIs
+ * Not detectable via API analysis
+
+### Category
+ASP.NET
+
+