Clarify 'unused identifier' rules in FAQ

I didn't understand this when I read this originally, and the penny didn't drop till I read the original issue discussion. I've reworded it to hopefully make it clearer.

docs/content/faq.fsx

@@ -48,10 +48,12 @@ If you encounter any bug, please take some time to record diagnostics informatio
 Why are object identifiers in members marked as unused?
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 If object identifiers (`x` in `x.Member`) aren't referred anywhere in the member bodies, they are understood as unused values.
-In order to fix it, we recommend to use `__` or any identifier with `_` prefix (`_x` for example).
+This *includes* the self-reference for class members (eg 'this').
+To suppress the warning, we recommend to use `__` (double underscore) or any identifier with `_` prefix (`_x` for example).
+For more info, see [issue 644](https://github.com/fsprojects/VisualFSharpPowerTools/issues/644)
 
 Why are my VFPT key bindings overridden by ReSharper?
 -----------------------
 If you are using Visual F# Power Tools along side with ReSharper 8 or below, there is a [known issue](http://youtrack.jetbrains.com/issue/RSRP-409199) where some F# commands are hidden by ReSharper. 
 You can workaround by disabling the ReSharper option "Hide overridden Visual Studio menu items" (ReSharper -> Options -> Keyboard & Menus).
-*)
+*)