doc: Some improvements in CREATE SUBSCRIPTION ref page

Add link to description of libpq connection strings.  Add link to
explanation of replication access control.  This currently points to the
description of streaming replication access control, which is currently
the same as for logical replication, but that might be refined later.
Also remove plain-text passwords from the examples, to not encourage
that dubious practice.

based on suggestions from Simon Riggs

doc/src/sgml/ref/create_subscription.sgml

@@ -76,7 +76,8 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
     <term><literal>CONNECTION '<replaceable class="parameter">conninfo</replaceable>'</literal></term>
     <listitem>
      <para>
-      The connection string to the publisher.
+      The connection string to the publisher.  For details
+      see <xref linkend="libpq-connstring">.
      </para>
     </listitem>
    </varlistentry>
@@ -126,6 +127,16 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
   </variablelist>
  </refsect1>
 
+ <refsect1>
+  <title>Notes</title>
+
+  <para>
+   See <xref linkend="streaming-replication-authentication"> for details on
+   how to configure access control between the subscription and the
+   publication instance.
+  </para>
+ </refsect1>
+
  <refsect1>
   <title>Examples</title>
 
@@ -136,7 +147,7 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
    commit:
 <programlisting>
 CREATE SUBSCRIPTION mysub
-         CONNECTION 'host=192.168.1.50 port=5432 user=foo dbname=foodb password=foopass'
+         CONNECTION 'host=192.168.1.50 port=5432 user=foo dbname=foodb'
         PUBLICATION mypublication, insert_only;
 </programlisting>
   </para>
@@ -147,7 +158,7 @@ CREATE SUBSCRIPTION mysub
    until enabled at a later time.
 <programlisting>
 CREATE SUBSCRIPTION mysub
-         CONNECTION 'host=192.168.1.50 port=5432 user=foo dbname=foodb password=foopass'
+         CONNECTION 'host=192.168.1.50 port=5432 user=foo dbname=foodb'
         PUBLICATION insert_only
                WITH (DISABLED);
 </programlisting>