Add missing documentation for the CryptographicOperations class. (#3476)

* Add missing documentation for the CryptographicOperations class.

* which -> that

* Apply suggestions from code review

Co-Authored-By: Genevieve Warren <gewarren@microsoft.com>

Author: 2 people

xml/System.Security.Cryptography/CryptographicOperations.xml

@@ -18,7 +18,7 @@
   </Base>
   <Interfaces />
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Provides methods for use in working with cryptography to reduce the risk of side-channel information leakage.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>
@@ -89,9 +89,18 @@ Fixed-time behavior is guaranteed in all other cases, including when `left` and
         <Parameter Name="buffer" Type="System.Span&lt;System.Byte&gt;" />
       </Parameters>
       <Docs>
-        <param name="buffer">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="buffer">The buffer to fill with zeros.</param>
+        <summary>Fills the provided buffer with zeros.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This method exists to future-proof against potential optimizations in the .NET runtime that could eliminate memory writes that aren't followed by memory reads.
+While the C and C++ compilers have similar optimizations, no such optimizations are planned at this time for .NET.
+
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
   </Members>