Add remarks about FakeTimeProvider to the description of TimeProvider. (#10256)

Author: voroninp

xml/System/TimeProvider.xml

@@ -33,7 +33,7 @@
   </Attributes>
   <Docs>
     <summary>Provides an abstraction for time.</summary>
-    <remarks>To be added.</remarks>
+    <remarks>To make it easier to test time-dependent code, you can use <see cref="T:Microsoft.Extensions.Time.Testing.FakeTimeProvider" /> from the Microsoft.Extensions.Time.Testing package.</remarks>
   </Docs>
   <Members>
     <Member MemberName=".ctor">