Reapply "nixos/postgresql: add docs about procedural languages"

This reverts commit a4006e4970.

nixos/doc/manual/redirects.json

@@ -1253,6 +1253,9 @@
   "module-services-postgres-plugins": [
     "index.html#module-services-postgres-plugins"
   ],
+  "module-services-postgres-pls": [
+    "index.html#module-services-postgres-pls"
+  ],
   "module-services-postgres-jit": [
     "index.html#module-services-postgres-jit"
   ],

nixos/modules/services/databases/postgresql.md

@@ -328,6 +328,37 @@ self: super: {
 }
 ```
 
+## Procedural Languages {#module-services-postgres-pls}
+
+PostgreSQL ships the additional procedural languages PL/Perl, PL/Python and PL/Tcl as extensions.
+They are packaged as plugins and can be made available in the same way as external extensions:
+```nix
+{
+  services.postgresql.extensions = ps: with ps; [
+    plperl
+    plpython3
+    pltcl
+  ];
+}
+```
+
+Each procedural language plugin provides a `.withPackages` helper to make language specific packages available at run-time.
+
+For example, to make `python3Packages.base58` available:
+```nix
+{
+  services.postgresql.extensions = pgps: with pgps; [
+    (plpython3.withPackages (pyps: with pyps; [ base58 ]))
+  ];
+}
+```
+
+This currently works for:
+- `plperl` by re-using `perl.withPackages`
+- `plpython3` by re-using `python3.withPackages`
+- `plr` by exposing `rPackages`
+- `pltcl` by exposing `tclPackages`
+
 ## JIT (Just-In-Time compilation) {#module-services-postgres-jit}
 
 [JIT](https://www.postgresql.org/docs/current/jit-reason.html)-support in the PostgreSQL package