Add remarks (#2779)

Provide remarks on the `GetFunctionPointer()` function.

Author: AaronRobinsonMSFT

xml/System/RuntimeMethodHandle.xml

@@ -200,7 +200,12 @@
       <Docs>
         <summary>Obtains a pointer to the method represented by this instance.</summary>
         <returns>A pointer to the method represented by this instance.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+## Remarks  
+   The returned <xref:System.IntPtr> is meant for internal runtime usage and should not be used to call the function from native code. Use <xref:System.Runtime.InteropServices.Marshal.GetFunctionPointerForDelegate%2A?displayProperty=nameWithType> in order to get a function pointer that can be passed to native code.  
+         ]]></format>
+        </remarks>
         <exception cref="T:System.Security.SecurityException">The caller does not have the necessary permission to perform this operation.</exception>
         <permission cref="T:System.Security.SecurityCriticalAttribute">Requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.</permission>
       </Docs>
@@ -442,4 +447,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>