Port type documentation for X509CertificateLoader (#10803)

Author: vcsjones

xml/System.Security.Cryptography.X509Certificates/X509CertificateLoader.xml

@@ -28,7 +28,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Provides methods for loading an X.509 certificate or a PKCS#12 PFX containing certificates.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>