website/docs: make file provisioner reqs clearer

Make extra clear the file provisioner needs destination directory to exist.

website/source/docs/provisioners/file.html.md

@@ -54,9 +54,11 @@ know.
 
 First, the destination directory must already exist. If you need to create it,
 use a shell provisioner just prior to the file provisioner in order to create
-the directory. Note that the `docker` builder does not have this requirement.
-It will create any needed destination directories, but it's generally best
-practice to not rely on this behavior.
+the directory. If the destination directory does not exist, the file
+provisioner may succeed, but it will have undefined results. Note that the
+`docker` builder does not have this requirement.  It will create any needed
+destination directories, but it's generally best practice to not rely on this
+behavior.
 
 Next, the existence of a trailing slash on the source path will determine
 whether the directory name will be embedded within the destination, or whether