-c can't reach all servers listed on a built-in directory

As @rdica [mentions here](https://github.com/jamulussoftware/jamuluswebsite/pull/1116#discussion_r2865450164), the -c command line option can't reach every server listed on a built-in directory. In fact, most servers might not be reachable using -c, because most clients rely on a "hole punch" procedure to reach a server without the server explicitly opening the port.

This probably merits mentioning in [the command-line documentation for this feature](https://jamulus.io/wiki/Software-Manual#command-line-options).

Open the linked Software Manual command-line options section and review the description of -c alongside the built-in directory behavior described in the issue. Document that -c cannot reach every listed server because many rely on hole punching; done means the limitation is clear to users in that section.

Written by the indexing model from the issue text.