archlinux installation doc: correct some details

1. The AUR package is called docker-git, not lxc-docker-git.

2. According to the official community package, docker depends on
   sqlite.

3. 02ef8ec (Update archlinux.rst as packages have changed, 2013-12-06)
   updated the installation instructions, but left behind residual
   wording about the AUR package not being officially supported; the
   community repository is officially supported.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>