docs/builder: add trailing slash condition for ADD .tar.gz

When `<dest>` doesn't end with a trailing `/`, I noticed that now (Server version: 1.1.0) `ADD foo.tar.gz dest` copy the tar to a file named `dest' instead of extracting. Clarifying the documention to be more explicit about this.
```
$ docker build .
Sending build context to Docker daemon 3.584 kB
Sending build context to Docker daemon 
Step 0 : FROM busybox
 ---> a9eb17255234
Step 1 : ADD foo.tar.gz /dest
 ---> Using cache
 ---> 66f077fdf0cc
Successfully built
$ docker run -ti 66f077fdf0cc
/ # ls -al dest
ls -al dest
-rw-r-----    1 root     root           156 Jul 14 23:17 dest
```