doc: clarify create database in start docs uses command line

Reported-by: vrms@netcologne.de

Discussion: https://postgr.es/m/172251463564.915373.17748961617119647662@wrigleys.postgresql.org

Backpatch-through: master

Author: bmomjian

doc/src/sgml/start.sgml

@@ -148,7 +148,7 @@
    </para>
 
    <para>
-    To create a new database, in this example named
+    To create a new database from the command line, in this example named
     <literal>mydb</literal>, you use the following command:
 <screen>
 <prompt>$</prompt> <userinput>createdb mydb</userinput>