Fix WebHeaderCollection header restrictions docs (#4972)

Fix WebHeaderCollection header restrictions docs according to its implementation.
Header restrictions are not checked since WebHeaderCollection was transferred to .NET Core.

Author: CarnaViire

xml/System.Net/WebHeaderCollection.xml

@@ -82,8 +82,10 @@
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The <xref:System.Net.WebHeaderCollection> class is generally accessed through <xref:System.Net.WebRequest.Headers%2A?displayProperty=nameWithType> or <xref:System.Net.WebResponse.Headers%2A?displayProperty=nameWithType>. Some common headers are considered restricted and are either exposed directly by the API (such as `Content-Type`) or protected by the system and cannot be changed.  
-  
+ The <xref:System.Net.WebHeaderCollection> class is generally accessed through <xref:System.Net.WebRequest.Headers%2A?displayProperty=nameWithType> or <xref:System.Net.WebResponse.Headers%2A?displayProperty=nameWithType>.
+
+ On .NET Framework, some common headers are considered restricted and are either exposed directly by the API (such as `Content-Type`) or protected by the system and cannot be changed. This does *not* apply to .NET Core and .NET 5+ - none of the headers are restricted there.
+
  The restricted headers are:  
   
 -   Accept