Fix documentation for Source.Span.Pos.

Fixes #457.

semantic-source/src/Source/Span.hs

@@ -1,4 +1,6 @@
-{-# LANGUAGE DeriveGeneric, OverloadedStrings, RankNTypes #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RankNTypes #-}
 -- | Source position and span information
 --
 --   Mostly taken from purescript's SourcePos definition.
@@ -56,7 +58,11 @@ spanFromSrcLoc :: SrcLoc -> Span
 spanFromSrcLoc s = Span (Pos (srcLocStartLine s) (srcLocStartCol s)) (Pos (srcLocEndLine s) (srcLocEndCol s))
 
 
--- | Source position information (1-indexed)
+-- | Source position information.
+-- The 'Pos' values associated with ASTs returned from tree-sitter
+-- 'Unmarshal' instances are zero-indexed. Unless you are displaying
+-- span information to a user, you should write your code assuming
+-- zero-indexing.
 data Pos = Pos
   { line   :: {-# UNPACK #-} !Int
   , column :: {-# UNPACK #-} !Int