Update documentation on RuntimeMethodHandle.GetFunctionPointer (dotnet#8061)

Co-authored-by: Genevieve Warren <[email protected]>

Author: jkoritzinsky

xml/System/RuntimeMethodHandle.xml

@@ -251,7 +251,10 @@
         <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.  
+ 
+ If the method has the <xref:System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute?displayProperty=nameWithType> attribute, then the returned value is an unmanaged function pointer with the same calling convention as specified in the attribute. If the method this handle represents is static, then the returned value can be cast to a C# managed function pointer type with the same signature. For instance method handles, the value is not easily usable from user code and is meant exclusively for usage within the runtime.
+   
+ For methods that don't have the <xref:System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute?displayProperty=nameWithType>, use <xref:System.Runtime.InteropServices.Marshal.GetFunctionPointerForDelegate%2A?displayProperty=nameWithType> 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>