Remove inbound links to sql-createuser

CREATE USER is an alias for CREATE ROLE, not its own command any longer,
so clean up references to the 'sql-createuser' link to go to
'sql-createrole' instead.

In passing, change a few cases of 'CREATE USER' to be
'CREATE ROLE ...  LOGIN'.  The remaining cases appear reasonable and
also mention the distinction between 'CREATE ROLE' and 'CREATE USER'.
Also, don't say CREATE USER "assumes" LOGIN, but rather "includes".

Patch-by: David G. Johnston, with assumes->includes by me.
Discussion: https://postgr.es/m/CAKFQuwYrbhKV8hH4TEABrDRBwf=gKremF=mLPQ6X2yGqxgFpYA@mail.gmail.com

doc/src/sgml/client-auth.sgml

@@ -998,9 +998,9 @@ omicron         bryanh                  guest1
     separate from operating system user passwords. The password for
     each database user is stored in the <literal>pg_authid</literal> system
     catalog. Passwords can be managed with the SQL commands
-    <xref linkend="sql-createuser"> and
+    <xref linkend="sql-createrole"> and
     <xref linkend="sql-alterrole">,
-    e.g., <userinput>CREATE USER foo WITH PASSWORD 'secret'</userinput>,
+    e.g., <userinput>CREATE ROLE foo WITH LOGIN PASSWORD 'secret'</userinput>,
     or the <application>psql</application>
     command <literal>\password</literal>.
     If no password has been set up for a user, the stored password

doc/src/sgml/ref/create_database.sgml

@@ -45,7 +45,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
   <para>
    To create a database, you must be a superuser or have the special
    <literal>CREATEDB</literal> privilege.
-   See <xref linkend="sql-createuser">.
+   See <xref linkend="sql-createrole">.
   </para>
 
   <para>

doc/src/sgml/user-manag.sgml

@@ -158,7 +158,7 @@ CREATE ROLE <replaceable>name</replaceable> LOGIN;
 CREATE USER <replaceable>name</replaceable>;
 </programlisting>
         (<command>CREATE USER</command> is equivalent to <command>CREATE ROLE</command>
-        except that <command>CREATE USER</command> assumes <literal>LOGIN</literal> by
+        except that <command>CREATE USER</command> includes <literal>LOGIN</literal> by
         default, while <command>CREATE ROLE</command> does not.)
        </para>
       </listitem>