Documentation: Add note that single-instance blogs are private

Hello! I had recently setup my own WriteFreely instance over at https://gperson.h0stname.net. It is intended to be used as my own personal public blog, so I naturally thought that having a single-user instance would be the way to go. However, I just recently discovered that having a single-user instance only allows your blog to be unlisted, private, or password protected. It turns out that, at the current moment, you need to have a multi-instance blog.

I have a simple solution to this problem: simply update the documentation to mention that you must have a multi-user instance in order to have a public blog. I intend to submit a PR improving the documentation to mention this too.

Steps to reproduce (if necessary)

Steps to reproduce the behavior:

1. Setup a WriteFreely instance with single_user = true under [app] in your config.
2. Visit blog settings and go to the publicity section.

Expected behavior

Initially, I thought that having a single user blog would be allowed to be public.

Application configuration

I don't think it matters, but just in case it does, here's my config:

• Single mode or Multi-user mode? Multi-user mode
• Database? SQLite
• Open registration? No
• Federation enabled? Yes

Version or last commit: 0.15.1, installed from the Alpine Linux package repositories.

Start by locating the documentation page covering single-user or multi-user setup and blog publicity, then review how the [app] single_user = true setting is described. Update it to state that public blogs require multi-user mode, and verify the wording against the publicity behavior described in this issue.