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/pocket.lua

@@ -0,0 +1,28 @@
+--[[-
+Control the current pocket computer, adding or removing upgrades.
+
+This API is only available on pocket computers. As such, you may use its
+presence to determine what kind of computer you are using:
+
+```lua
+if pocket then
+  print("On a pocket computer")
+else
+  print("On something else")
+end
+```
+]]
+
+--- Search the player's inventory for another upgrade, replacing the existing
+-- one with that item if found.
+--
+-- This inventory search starts from the player's currently selected slot,
+-- allowing you to prioritise upgrades.
+--
+-- @throws If an upgrade cannot be found.
+function equipBack() end
+
+--- Remove the pocket computer's current upgrade.
+--
+-- @throws If this pocket computer does not currently have an upgrade.
+function unequipBack() end