Correct documentation of FS move

README.md

@@ -154,9 +154,15 @@ contained files and directories, without following symbolic links.
 
 ### move(source, target)
 
-Moves a file or directory from one path to another.  If the target
-exists and is a directory, moves the source into the target directory
-with the same base name as the source.
+Moves a file or directory from one path to another.  Cannot move over a
+target directory, even if it is empty.  Otherwise, does nothing if the
+source and target are the same entry in the file system.
+
+Node's `rename` implementation, at least on Mac OS X, does not enforce
+the rule that writing over an empty directory should fail.  Since Q-IO
+enforces this rule in software, it is not atomic and there is a chance
+that an empty directory will be created over the target path between
+when `move` checks for it and when it overwrites it.
 
 ### link(source, taget)