Update documentation regarding repeating with counts

Repeating commands with counts works since Vim 7.4.160. Example:

```vim
a, b, c, d, e, f
a, b, c, d, e, f
```

Execute `d3an,` and repeat on next line with `+.` leads to.

```vim
a, b, c, e, f
a, b, c, e, f
```