Remove documentation of SecurityMitigations field (dotnet#4652)

* Update MethodImplOptions.xml

* Update MethodImplAttributes.xml

Author: MichalStrehovsky

xml/System.Reflection/MethodImplAttributes.xml

@@ -728,7 +728,7 @@
       </ReturnValue>
       <MemberValue>1024</MemberValue>
       <Docs>
-        <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>
+        <summary>This member is reserved for future use. It was introduced in .NET Framework 4.8.</summary>
       </Docs>
     </Member>
     <Member MemberName="Synchronized">

xml/System.Runtime.CompilerServices/MethodImplOptions.xml

@@ -347,7 +347,7 @@
       </ReturnValue>
       <MemberValue>1024</MemberValue>
       <Docs>
-        <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>
+        <summary>This member is reserved for future use. It was introduced in .NET Framework 4.8.</summary>
       </Docs>
     </Member>
     <Member MemberName="Synchronized">