Docs: Docker doesn't support pub/priv networks

The Docker provider ignores the network identifiers `private_network` and
`public_network`. Document this fact, because it's not immediately obvious.
Merge pull request #5053 from dcarley/doc_docker_pub_priv_networks

Docs: Docker doesn't support pub/priv networks
Make completion.sh respect VAGRANT_HOME


When VAGRANT_HOME is set to a different path than $HOME/.vagrant.d, bash
completions regarding box names fails with an error like:

"$ vagrant init <TAB> find: `/home/mert/.vagrant.d/boxes': No such file or directory"

This commit makes completion.sh respect VAGRANT_HOME if it is set. It will continue to use $HOME/.vagrant.d otherwise.
Merge pull request #5055 from mdirik/master

bash completion doesn't respect VAGRANT_HOME