docs: flesh out the process for updating a package. (#1186)

* docs: flesh out the process for updating a package.

* docs: include bit abvout buildPgrxExtension previousVersions

---------

Co-authored-by: Sam Rose <[email protected]>

Author: samrose

nix/docs/update-extension.md

@@ -6,9 +6,12 @@
 2. For instance, if we were updating https://github.com/supabase/postgres/blob/develop/nix/ext/supautils.nix we would:
    1. change the `version = "2.2.1";` to whatever our git tag release version is that we want to update to
    2. temporarily empty the `hash = "sha256-wSUEG0at00TPAoHv6+NMzuUE8mfW6fnHH0MNxvBdUiE=";` to `hash = "";` and save `supautils.nix` and `git add  .`
-   3. run `nix build .#psql_15/exts/supautils` or the name of the extension to update, nix will print the calculated sha256 value that you can add back the the `hash` variable, save the file again, and re-run nix build .#psql_15/exts/supautils. 
-   4. Add any needed migrations into the `supabase/postgres` migrations directory
-   5. You can then run tests locally to verify that the update of the package succeeded. 
-   6. Now it's ready for PR review.
+   3. run `nix build .#psql_15/exts/supautils` or the name of the extension to update, nix will print the calculated sha256 value that you can add back the the `hash` variable, save the file again, and re-run nix build .#psql_15/exts/supautils.
+   4. NOTE: This step is only necessary for `buildPgrxExtension` packages, which includes supabase-wrappers, pg_jsonschema, and pg_graphql. Otherwise you can skip this step. For our packages that are build with `buildPgrxExtension` you will need to prepend the previous version to the `previousVersions` variable before updating the version in the package (for instance if you are updating `supabase-wrappers` extension from `0.4.1` to `0.4.2` then you would prepend `0.4.1` to this line https://github.com/supabase/postgres/blob/develop/nix/ext/wrappers/default.nix#L18 ). 
+   5. Add any needed migrations into the `supabase/postgres` migrations directory
+   6. update the version in `ansible/vars.yml` as usual
+   7. You can then run the `nix flake check -L` tests locally to verify that the update of the package succeeded. 
+   8. Now it's ready for PR review.
+   9. Once the PR is approved, if you want the change to go out in a release, update the common-nix.vars.yml file with the new version prior to merging.