Update Assembly.IsFullyTrusted and AppDomain.IsFullyTrusted properties for .NET Core/5+ behaviour (dotnet#10343)

Author: elinor-fung

xml/System.Reflection/Assembly.xml

@@ -3901,7 +3901,10 @@ This property is marked obsolete starting in .NET 5, and generates a compile-tim
       <Docs>
         <summary>Gets a value that indicates whether the current assembly is loaded with full trust.</summary>
         <value>
-          <see langword="true" /> if the current assembly is loaded with full trust; otherwise, <see langword="false" />.</value>
+          .NET Framework only: <see langword="true" /> if the current assembly is loaded with full trust; otherwise, <see langword="false" />.
+
+          .NET Core and .NET 5+: <see langword="true" /> in all cases.
+        </value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -6328,9 +6331,7 @@ In .NET 5 and later versions, for bundled assemblies, the value returned is an e
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
- By default, assemblies that you compile with the .NET Framework 4 have [level 2](/dotnet/framework/misc/security-transparent-code-level-2) transparency, although you can explicitly make them [level 1](/dotnet/framework/misc/security-transparent-code-level-1) instead. Assemblies that were compiled with earlier versions of the .NET Framework have level 1 transparency.
-
- See [Security Changes](/dotnet/framework/security/security-changes).
+ **.NET Framework only:** By default, assemblies that you compile with the .NET Framework 4 have [level 2](/dotnet/framework/misc/security-transparent-code-level-2) transparency, although you can explicitly make them [level 1](/dotnet/framework/misc/security-transparent-code-level-1) instead. Assemblies that were compiled with earlier versions of the .NET Framework have level 1 transparency. See [Security Changes](/dotnet/framework/security/security-changes).
 
  ]]></format>
         </remarks>

xml/System/AppDomain.xml

@@ -6509,12 +6509,15 @@ This method overload uses the <xref:System.AppDomainSetup> information from the
       <Docs>
         <summary>Gets a value that indicates whether assemblies that are loaded into the current application domain execute with full trust.</summary>
         <value>
-          <see langword="true" /> if assemblies that are loaded into the current application domain execute with full trust; otherwise, <see langword="false" />.</value>
+          .NET Framework only: <see langword="true" /> if assemblies that are loaded into the current application domain execute with full trust; otherwise, <see langword="false" />.
+
+          .NET Core and .NET 5+: <see langword="true" /> in all cases.
+        </value>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
- This method always returns `true` for the default application domain of an application that runs on the desktop. It returns `false` for a sandboxed application domain that was created by using the <xref:System.AppDomain.CreateDomain%28System.String%2CSystem.Security.Policy.Evidence%2CSystem.AppDomainSetup%2CSystem.Security.PermissionSet%2CSystem.Security.Policy.StrongName%5B%5D%29?displayProperty=nameWithType> method overload, unless the permissions that are granted to the application domain are equivalent to full trust.
+ **.NET Framework only:** This method always returns `true` for the default application domain of an application that runs on the desktop. It returns `false` for a sandboxed application domain that was created by using the <xref:System.AppDomain.CreateDomain%28System.String%2CSystem.Security.Policy.Evidence%2CSystem.AppDomainSetup%2CSystem.Security.PermissionSet%2CSystem.Security.Policy.StrongName%5B%5D%29?displayProperty=nameWithType> method overload, unless the permissions that are granted to the application domain are equivalent to full trust.