DOC: config: Fix bind/server/peer documentation in the peers section

DOC: config: Fix bind/server/peer documentation in the peers section

Documentation about bind and server directives in the peers section was
retrieved from the proxy part but there are some limitations, especially for
the bind directive. And the same is true for the peer directive. It is
forbidden to have several listening addresses. Multiple addresses or port
range are not allowed.

Here, only the documentation is fixed. The configuration parsing will be
improved later to trigger errors on bad uses.

In addition, it is also specified that unix socket are supported.

This patch partially fixes the issue #2066. It should be backported to all
stable versions.