Documentation: explain what's included in the all alias

[jchavarri]

Afaik, the docs don't explain what kind of targets are included in the all alias: https://dune.readthedocs.io/en/stable/reference/aliases.html#built-in-aliases.

It would be great for users to get an idea of what exactly is being built there. So far, I know:

• executable stanzas
• public libraries
• melange.emit stanzas (see related PR melange - add melange.emit targets to @all alias as well by default #7926)
• ???