Give directions on correct usage of pg_guard and similar

[workingjubilee]

For a few fundamentally unsafe declarations, the ideal size should be as small as possible. There will be a similar function on MemCx. In the guide, we will need to explain code organization on a high level, and saying "really, try to make these as small as possible. pg_guard is for pgrx-pg-sys and certain callback-driven APIs, use pg_extern for anything you want to expose through an extension" is useful.

[workingjubilee]

We also could be up-front that the pg_extern annotations create an extremely thick shell function boundary to handle the Postgres Function Info V1 ABI, and that is why pg_guard functions have more restrictions! They don't get the protective wrappers and they aren't expected to handle the Postgres ABI.