Update HAProxy reverse proxy documentation to include support for unix domain sockets

[kmartens27]

Describe your use-case which is not covered by existing documentation.

As a result of merging this packaging pull request, Jenkins now supports Unix domain sockets.

The various reverse proxy configuration pages need to be updated accordingly to include this new information. This ticket is for the HAProxy reverse proxy specifically, as other configurations will have their own tickets.

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

No response