Add X509CertificateLoader documentation

Author: vcsjones

src/libraries/Common/src/System/Security/Cryptography/X509Certificates/X509CertificateLoader.cs

@@ -12,10 +12,11 @@
 
 namespace System.Security.Cryptography.X509Certificates
 {
+    /// <summary>
+    ///   Provides methods for loading an X.509 certificate or a PKCS#12 PFX containing certificates.
+    /// </summary>
     [UnsupportedOSPlatform("browser")]
-#pragma warning disable 1591 // TODO: Document this API. https://github.com/dotnet/runtime/issues/105974
     public static partial class X509CertificateLoader
-#pragma warning restore 1591
     {
         private const int MemoryMappedFileCutoff = 1_048_576;