doc: Add note to pg_dump man page about pg_dumpall

suggested by Joshua Drake

doc/src/sgml/ref/pg_dump.sgml

@@ -45,6 +45,12 @@ PostgreSQL documentation
    accessing the database (readers or writers).
   </para>
 
+  <para>
+   <application>pg_dump</application> only dumps a single database.  To backup
+   global objects that are common to all databases in a cluster, such as roles
+   and tablespaces, use <xref linkend="app-pg-dumpall">.
+  </para>
+
   <para>
    Dumps can be output in script or archive file formats. Script
    dumps are plain-text files containing the SQL commands required