Clarify behaviour of 'secret server variables' in SSG mode

[thegatesdev]

https://docs.astro.build/en/guides/environment-variables/#variable-types
On this page, the different types of environment variables are explained.
Both 'public client' and 'public server' variables make sense in SSG mode, they just get replaced by Vite at build time.
However, the behaviour of 'secret server' variables is not clear for that mode.
Do they:

• also get replaced (doesn't make sense and would be unsafe)
• generate an error (a mention that this type does not work on SSG would be nice then)
• not get replaced (idem to above, needs a mention in the docs)

Thanks!