Write docs for v12.2.0 minor release

[wolfgangwalther]

This is already listed in #3113 (comment), but to give this more visibility I'll create an issue about it.

Ever since merging the postgrest-docs repo into the core repo, we are supposed to merge docs together in the same PR as the corresponding feature. This didn't always happen initially, but we are getting better at it. However, we still have some unreleased feature commits on the main branch, which happened before the docs merge and thus have no corresponding docs written, yet.

This is blocking the next minor release, so needs to be taken care of.

I went through the changelog on main again, and I think the following still need to be documented:

• Dump config through Admin API #3171, Add an ability to dump config via admin API - @SkyWriter
• Dump Schema Cache through Admin API #3210, Dump schema cache through admin API - @taimoorzaeem
• Deprecated: Prefer: params=single-object is deprecated. Use a function with a single unnamed JSON parameter instead. - @steve-chavez

@steve-chavez since you merged the first two and wrote the last yourself - can you take care of the docs for those to unblock a release?

There is also #3061, which is still undocumented, but also blocking a new minor release is #3242 which will be fixed in #3358. This essentially contains the docs for #3061, too.