doc(lexer): explain the use of incSourceColumn

I think it is clearer to phrase it this way so it is clear that we are
not unconsuming (i.e. changing the state of the parser).

src/Lexer.hs

@@ -221,7 +221,7 @@ link :: Lexer
 link = do
     pos <- getPosition
     l <- linkRaw
-    -- "unconsume" the last token
+    -- register the position of the last token
     pos' <- flip incSourceColumn (-1) <$> getPosition
     pure $ (pos, LinkOpen) : l <> [(pos', LinkClose)]