Fix usages of global variables

- Lint references to unknown fields of modules, excluding the keys and
   colours modules. This caught several silly errors in our stub files,
   but nothing else.
 - Lint on using unknown globals. This highlighted a couple of really
   silly mistakes. Fixes #427.
 - Add documentation for fs.attributes, fs.getCapacity and pocket, as
   they were not defined before.

Co-authored-by: JackMacWindows <jackmacwindowslinux@gmail.com>

doc/stub/os.lua

@@ -15,3 +15,10 @@ function cancelTimer(id) end
 function cancelAlarm(id) end
 function epoch(timezone) end
 function date(format, time) end
+
+-- Defined in bios.lua
+function loadAPI(path) end
+function pullEvent(filter) end
+function pullEventRaw(filter) end
+function version() end
+function run(env, path, ...) end