Add note that SafeCanonicalizationMethods is static-like. (dotnet#6712)

Author: vcsjones

xml/System.Security.Cryptography.Xml/SignedXml.xml

@@ -1324,6 +1324,9 @@ The main parts of this structure are:
 ## Remarks  
  Canonicalization methods convert an XML document to its canonical form. For more information about canonical XML, see [Canonical XML](https://www.w3.org/TR/xml-c14n) on the W3C website.  
   
+> [!NOTE]
+>  <xref:System.Security.Cryptography.Xml.SignedXml.SafeCanonicalizationMethods%2A> behaves like a static property. Changes from one instance of <xref:System.Security.Cryptography.Xml.SignedXml> will be observed in other instances. Modifying the collection is not a thread-safe operation.
+
  ]]></format>
         </remarks>
       </Docs>