_PG_fini has been removed entirely in 15

[kovmir]

THANK YOU FOR THE HARD WORK!

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=ab02d702ef08343fba30d90fdf7df5950063e8c9

So I think you should alter this paragraph in Detailed.md.

To pop the state of the hook created by one extension `_PG_fini` function must be implemented, 
which is basically recovers hook to it's value before `_PG_init`.

[kovmir]

By the way these _PG_fini() functions have never actually done anything https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/fmgr/dfmgr.c;hb=refs/heads/REL_14_STABLE#l388