Improve the instructions for PgSQL setup

- There's no such option as "-A"
- It's better to pass -S (for --no-superuser) and -R (--no-createrole) than to
be prompted for them interactively.

Change-Id: I0b10a6c6e67a4362f510c022126354e8d6254e62

Documentation/database-setup.txt

@@ -33,7 +33,7 @@ password, create a database to store the metadata, and grant the user
 full rights on the newly created database:
 
 ----
-  $ createuser --username=postgres -A -D -P -E gerrit2
+  $ createuser --username=postgres -S -R -D -P -E gerrit2
   $ createdb --username=postgres -E UTF-8 -O gerrit2 reviewdb
 ----