Add a note about Alpine location of "postgresql.conf.sample"

Fixes docker-library/postgres#423

postgres/content.md

@@ -167,7 +167,7 @@ Since database initialization only happens on container startup, this allows us
 
 There are many ways to set PostgreSQL server configuration. For information on what is available to configure, see the postgresql.org [docs](https://www.postgresql.org/docs/current/static/runtime-config.html) for the specific version of PostgreSQL that you are running. Here are a few options for setting configuration:
 
--	Use a custom config file. Create a config file and get it into the container. If you need a starting place for your config file you can use the sample provided by PostgreSQL which is available in the container at `/usr/share/postgresql/postgresql.conf.sample`.
+-	Use a custom config file. Create a config file and get it into the container. If you need a starting place for your config file you can use the sample provided by PostgreSQL which is available in the container at `/usr/share/postgresql/postgresql.conf.sample` (`/usr/local/share/postgresql/postgresql.conf.sample` in Alpine variants).
 
 	-	**Important note:** you must set `listen_addresses = '*'`so that other containers will be able to access %%REPO%%.