Add clarification around Set-AzWebAppSlot -AppSettings

Also includes changes to ChangeLog

Author: richardszalay

src/Websites/Websites/ChangeLog.md

@@ -19,6 +19,7 @@
 -->
 ## Upcoming Release
 * Fixed miscellaneous typos across module
+* Add clarification around -AppSettings parameter in Set-AzWebApp and Set-AzWebAppSlot
 
 ## Version 1.4.0
 * Fixing a bug where some SiteConfig properties were not returned by Get-AzWebApp and Set-AzWebApp

src/Websites/Websites/help/Set-AzWebAppSlot.md

@@ -78,7 +78,7 @@ Accept wildcard characters: False
 ```
 
 ### -AppSettings
-App Settings HashTable
+App Settings HashTable. Existing App Settings will be replaced, removing any settings that are not provided.
 
 ```yaml
 Type: System.Collections.Hashtable