Document LINQ to XML thread safety

Explain that concurrent reads are safe when no thread modifies the XObject, XDocument, or XElement instance.

Fix dotnet/runtime#123068

Author: 33modeling

xml/System.Xml.Linq/XDocument.xml

@@ -55,6 +55,10 @@
 ## Remarks
  For details about the valid content of an <xref:System.Xml.Linq.XDocument>, see [Valid Content of XElement and XDocument Objects](/dotnet/standard/linq/valid-content-xelement-xdocument-objects).
 
+## Thread safety
+
+ An <xref:System.Xml.Linq.XDocument> instance is safe for concurrent read operations as long as no thread modifies it. If the instance is modified, you must synchronize all access to it.
+
 
 
 ## Examples

xml/System.Xml.Linq/XElement.xml

@@ -85,6 +85,10 @@
 
  Some <xref:System.Xml.Linq.XElement> methods can be used from XAML. For more information, see [LINQ to XML Dynamic Properties](/dotnet/desktop/wpf/data/linq-to-xml-dynamic-properties).
 
+## Thread safety
+
+ An <xref:System.Xml.Linq.XElement> instance is safe for concurrent read operations as long as no thread modifies it. If the instance is modified, you must synchronize all access to it.
+
 
 
 ## Examples

xml/System.Xml.Linq/XObject.xml

@@ -61,6 +61,10 @@
 
  Note that annotations are not part of the XML infoset; they are not serialized or deserialized.
 
+## Thread safety
+
+ An <xref:System.Xml.Linq.XObject> instance, including an instance of a derived type, is safe for concurrent read operations as long as no thread modifies it. If the instance is modified, you must synchronize all access to it.
+
  ]]></format>
     </remarks>
     <related type="Article" href="/dotnet/standard/linq/linq-xml-overview">LINQ to XML overview</related>