docs(nix): clarify how a glibc bump affects collation versions

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: brainrake

flake.nix

@@ -27,8 +27,8 @@
     # for extensions that require older package versions
     nixpkgs-oldstable.url = "github:NixOS/nixpkgs/a76c4553d7e741e17f289224eda135423de0491d";
     # Postgres, its extensions, and anything else loaded into the postgres
-    # process stay on glibc 2.40. A glibc bump changes the libc collation
-    # version of every existing database.
+    # process stay on glibc 2.40. Postgres warns when the running glibc's
+    # collation version differs from the one stored in each database.
     nixpkgs-pg17.url = "https://releases.nixos.org/nixos/unstable/nixos-26.05pre921484.fb7944c166a3/nixexprs.tar.xz?rev=fb7944c166a3b630f177938e478f0378e64ce108";
     nixpkgs.url = "https://channels.nixos.org/nixos-26.05/nixexprs.tar.xz";
     rust-overlay.inputs.nixpkgs.follows = "nixpkgs";

nix/docs/updating-dependencies.md

@@ -72,7 +72,7 @@ Similar patterns can be followed for other dependencies defined in the nix packa
 
 The flake uses two nixpkgs inputs:
 
-- `nixpkgs-pg17` pins the nixpkgs that builds Postgres, every extension, and anything loaded into the postgres process, such as `supabase-groonga` and the `gatekeeper` PAM module. It pins glibc 2.40. A glibc change bumps the libc collation version of every existing database, so leave this input alone unless you plan that migration. Code built against a newer glibc fails to load into a postgres process running an older one.
+- `nixpkgs-pg17` pins the nixpkgs that builds Postgres, every extension, and anything loaded into the postgres process, such as `supabase-groonga` and the `gatekeeper` PAM module. It pins glibc 2.40. After a glibc change, Postgres warns that the running collation version differs from the one stored in each database. Leave this input alone unless you plan that migration. Code built against a newer glibc fails to load into a postgres process running an older one.
 - `nixpkgs` tracks the current NixOS stable channel and builds everything else. Update it with `nix flake update nixpkgs`. When the stable release changes, also move `nix-darwin` to the matching `nix-darwin-YY.MM` branch.
 
 After an update, check that every Postgres output keeps its drvPath, for example `nix eval --raw .#packages.x86_64-linux."psql_17/bin".drvPath`.