Update postgres initdb information about sourcing scripts vs executing them

for https://github.com/docker-library/postgres/pull/452

postgres/content.md

@@ -136,7 +136,7 @@ The three easiest ways to get around this:
 
 # How to extend this image
 
-If you would like to do additional initialization in an image derived from this one, add one or more `*.sql`, `*.sql.gz`, or `*.sh` scripts under `/docker-entrypoint-initdb.d` (creating the directory if necessary). After the entrypoint calls `initdb` to create the default `postgres` user and database, it will run any `*.sql` files and source any `*.sh` scripts found in that directory to do further initialization before starting the service.
+If you would like to do additional initialization in an image derived from this one, add one or more `*.sql`, `*.sql.gz`, or `*.sh` scripts under `/docker-entrypoint-initdb.d` (creating the directory if necessary). After the entrypoint calls `initdb` to create the default `postgres` user and database, it will run any `*.sql` files, run any executable `*.sh` scripts, and source any non-executable `*.sh` scripts found in that directory to do further initialization before starting the service.
 
 For example, to add an additional user and database, add the following to `/docker-entrypoint-initdb.d/init-user-db.sh`: