Documented some new 4.8 APIs (#2253)

* Documented some new 4.8 APIs

* Corrected bad links

* Update xml/System.Threading/Interlocked.xml

Co-Authored-By: rpetrusha <[email protected]>

Author: Ron Petrusha

xml/System.Reflection/MethodImplAttributes.xml

@@ -83,9 +83,11 @@
 -   `NoOptimization`  
   
 -   `MaxMethodImplVal`  
-  
+
+- `SecurityMitigations` (Available starting with .NET Framework 4.8.)
+    
 > [!NOTE]
->  Locking on the instance or on the type, as with the Synchronized flag, is not recommended for public types, because code other than your own can take locks on public types and instances. This might cause deadlocks or other synchronization problems.  
+>  Locking on the instance or on the type, as with the `Synchronized` flag, is not recommended for public types, because code other than your own can take locks on public types and instances. This might cause deadlocks or other synchronization problems.  
   
  ]]></format>
     </remarks>
@@ -681,7 +683,7 @@
       </ReturnValue>
       <MemberValue>1024</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Specifies that the JIT compiler should look for security mitigation attributes, such as the user-defined <see langword="System.Runtime.CompilerServices.SecurityMitigationsAttribute" />. If found, the JIT compiler applies any related security mitigations. Available starting with .NET Framework 4.8.</summary>
       </Docs>
     </Member>
     <Member MemberName="Synchronized">

xml/System.Runtime.CompilerServices/MethodImplOptions.xml

@@ -322,7 +322,7 @@
       </ReturnValue>
       <MemberValue>1024</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The JIT compiler should look for security mitigation attributes, such as the user-defined <see langword="System.Runtime.CompilerServices.SecurityMitigationsAttribute" />. If found, the JIT compiler applies any related security mitigations. Available starting with .NET Framework 4.8.</summary>
       </Docs>
     </Member>
     <Member MemberName="Synchronized">

xml/System.Threading/Interlocked.xml

@@ -1559,9 +1559,15 @@ This method wraps a call to [FlushProcessWriteBuffers](/windows/desktop/api/proc
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <summary>Defines a memory fence that blocks speculative execution past this point until pending reads and writes are complete.</summary>
+        <remarks>  
+          <format type="text/markdown"><![CDATA[  
+
+The `SpeculationBarrier` method defines a memory fence (an `mfence` instruction on x86 and amd64 processors) that blocks *speculative* execution past this point until pending reads and writes are complete. 
+
+ ]]></format>
+        </remarks> 
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>