I recommend having an include statement at the top of every file that imports whatever the file uses directly. The reason is that the extension only considers each file separately (and anything it includes, recursively) when validating it.
Otherwise, adding all as default included as Emily said is the only other solution.
Your screenshot is not enough information to say anything more.
Currently I'm working on getting all internal functions (global functions, Game->/Screen->/bitmap-> etc. methods/properties) to provide a documentation comment when hovering over them (currently only have that working for stuff you define in a script yourself).
(3.0 only) This is implemented now.
You can hover over any of the internal functions or global variables to get a documentation comment as a tooltip (if present). You can also Ctrl/Cmd click on these internal symbols to open the relevant "bindings" file, which shows you all the other methods available for that specific internal class.