Update PostgreSQL documentation now that 18 is GA

This adjusts some examples to use `/var/lib/postgresql` as appropriate, and dials in some other language to be more focused on the 18+ reality.

postgres/content.md

@@ -102,20 +102,7 @@ Users who wish to opt-in to this change on older releases can do so by setting `
 
 > **Important Note:** (for PostgreSQL 17 and below) Mount the data volume at `/var/lib/postgresql/data` and not at `/var/lib/postgresql` because mounts at the latter path WILL NOT PERSIST database data when the container is re-created. The Dockerfile that builds the image declares a volume at `/var/lib/postgresql/data` and if no data volume is mounted at that path then the container runtime will automatically create an [anonymous volume](https://docs.docker.com/engine/storage/#volumes) that is not reused across container re-creations. Data will be written to the anonymous volume rather than your intended data volume and won't persist when the container is deleted and re-created.
 
-This optional variable can be used to define another location - like a subdirectory - for the database files. The default is `/var/lib/postgresql/data`. If the data volume you're using is a filesystem mountpoint (like with GCE persistent disks), or remote folder that cannot be chowned to the `postgres` user (like some NFS mounts), or contains folders/files (e.g. `lost+found`), Postgres `initdb` requires a subdirectory to be created within the mountpoint to contain the data.
-
-For example:
-
-```console
-$ docker run -d \
-	--name some-postgres \
-	-e POSTGRES_PASSWORD=mysecretpassword \
-	-e PGDATA=/var/lib/postgresql/data/pgdata \
-	-v /custom/mount:/var/lib/postgresql/data \
-	%%IMAGE%%
-```
-
-This is an environment variable that is not Docker specific. Because the variable is used by the `postgres` server binary (see the [PostgreSQL docs](https://www.postgresql.org/docs/14/app-postgres.html#id-1.9.5.14.7)), the entrypoint script takes it into account.
+This (`PGDATA`) is an environment variable that is not Docker specific. Because the variable is used by the `postgres` server binary (see the [PostgreSQL docs](https://www.postgresql.org/docs/14/app-postgres.html#id-1.9.5.14.7)), the entrypoint script takes it into account.
 
 ## Docker Secrets
 
@@ -204,7 +191,7 @@ When using the Alpine variants, any postgres extension not listed in [postgres-c
 
 As of [docker-library/postgres#253](https://github.com/docker-library/postgres/pull/253), this image supports running as a (mostly) arbitrary user via `--user` on `docker run`. As of [docker-library/postgres#1018](https://github.com/docker-library/postgres/pull/1018), this is also the case for the Alpine variants.
 
-The main caveat to note is that `postgres` doesn't care what UID it runs as (as long as the owner of `/var/lib/postgresql/data` matches), but `initdb` *does* care (and needs the user to exist in `/etc/passwd`):
+The main caveat to note is that `postgres` doesn't care what UID it runs as (as long as the owner of `PGDATA` matches), but `initdb` *does* care (and needs the user to exist in `/etc/passwd`):
 
 ```console
 $ docker run -it --rm --user www-data -e POSTGRES_PASSWORD=mysecretpassword %%IMAGE%%
@@ -230,13 +217,13 @@ The three easiest ways to get around this:
 3.	initialize the target directory separately from the final runtime (with a `chown` in between):
 
 	```console
-	$ docker volume create pgdata
-	$ docker run -it --rm -v pgdata:/var/lib/postgresql/data -e POSTGRES_PASSWORD=mysecretpassword %%IMAGE%%
+	$ docker volume create pg
+	$ docker run -it --rm -v pg:/var/lib/postgresql -e POSTGRES_PASSWORD=mysecretpassword %%IMAGE%%
 	The files belonging to this database system will be owned by user "postgres".
 	...
 	( once it's finished initializing successfully and is waiting for connections, stop it )
-	$ docker run -it --rm -v pgdata:/var/lib/postgresql/data bash chown -R 1000:1000 /var/lib/postgresql/data
-	$ docker run -it --rm --user 1000:1000 -v pgdata:/var/lib/postgresql/data %%IMAGE%%
+	$ docker run -it --rm -v pg:/var/lib/postgresql bash chown -R 1000:1000 /var/lib/postgresql
+	$ docker run -it --rm --user 1000:1000 -v pg:/var/lib/postgresql %%IMAGE%%
 	LOG:  database system was shut down at 2017-01-20 00:03:23 UTC
 	LOG:  MultiXact member wraparound protections are now enabled
 	LOG:  autovacuum launcher started
@@ -262,7 +249,7 @@ The Docker documentation is a good starting point for understanding the differen
 2.	Start your `%%IMAGE%%` container like this:
 
 	```console
-	$ docker run --name some-%%REPO%% -v /my/own/datadir:/var/lib/postgresql/data -e POSTGRES_PASSWORD=mysecretpassword -d %%IMAGE%%:tag
+	$ docker run --name some-%%REPO%% -v /my/own/datadir:/var/lib/postgresql -e POSTGRES_PASSWORD=mysecretpassword -d %%IMAGE%%:tag
 	```
 
-The `-v /my/own/datadir:/var/lib/postgresql/data` part of the command mounts the `/my/own/datadir` directory from the underlying host system as `/var/lib/postgresql/data` inside the container, where PostgreSQL by default will write its data files.
+The `-v /my/own/datadir:/var/lib/postgresql` part of the command mounts the `/my/own/datadir` directory from the underlying host system as `/var/lib/postgresql` inside the container, where PostgreSQL by default will write its data files.