Warnings updates (documentationjs#413)

* Remove MultipleFFIModules documentation

It is a template file and the error was removed in 0.12.0

* Add UnusedDeclaration warning docs

Author: nwolverson

errors/MultipleFFIModules.md

@@ -0,0 +1,26 @@
+# `UnusedDeclaration` Warning
+
+## Example
+
+```purescript
+module ShortFailingExample ( exportedThing ) where
+
+-- This declaration is exported, so used
+exportedThing = 42 :: Int
+
+-- This declaration is neither used in the module, nor exported
+problem = "problem" :: String
+
+```
+
+## Cause
+
+This warning identifies a declaration that is not exported, and is not used in the containing module. Such a declaration is "dead code" and can safely be removed.
+
+## Fix
+
+- Remove the declaration if it is unused
+- Use it within the module
+- Export the declaration if required
+
+## Notes