Improve TextWriter.NewLine docs (dotnet#8724)

* Improve TextWriter.NewLine docs, fixes dotnet#1477

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

Author: adamsitnik

xml/System.IO/TextWriter.xml

@@ -847,7 +847,8 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The default line terminator string is a carriage return followed by a line feed ("\r\n").  
+
+For non-Unix platforms, the default line terminator string is a carriage return followed by a line feed ('\r\n'). For Unix platforms, it's a line feed ('\n').  
   
  The line terminator string is written to the text stream whenever one of the `WriteLine` methods is called. In order for text written by the `TextWriter` to be readable by a <xref:System.IO.TextReader>, only "\n" or "\r\n" should be used as terminator strings. If `NewLine` is set to `null`, the default newline character is used instead.