Detail enhance description of AllowUnknownCertificateAuthority

AllowUnknownCertificateAuthority not only ignores untrusted roots, but also partial chains. This updates the documentation to reflect this behavior.

Author: stewartadam

xml/System.Security.Cryptography.X509Certificates/X509VerificationFlags.xml

@@ -131,7 +131,7 @@
       </ReturnValue>
       <MemberValue>16</MemberValue>
       <Docs>
-        <summary>Ignore that the chain cannot be verified due to an unknown certificate authority (CA).</summary>
+        <summary>Ignore that the chain cannot be verified due to an unknown certificate authority (CA) or partial chains.</summary>
       </Docs>
     </Member>
     <Member MemberName="IgnoreCertificateAuthorityRevocationUnknown">