doc: Clarify where ereport severity levels are defined

For a reader unfamiliar with the postgres code it might take some
grepping to find where elevels are defined. This adds a reference
to elog.h in the text like how SQLSTATE errorcodes are referenced
to errcodes.h on the same page.

Author: Kuwamura Masaki <kuwamura@db.is.i.nagoya-u.ac.jp>
Discussion: https://postgr.es/m/CAMyC8qqp1UDA9zothnJ9CbUYByytwpALS3LkdZ6bs1w5kZw5Xg@mail.gmail.com

doc/src/sgml/sources.sgml

@@ -105,7 +105,8 @@ less -x4
 
    <para>
     There are two required elements for every message: a severity level
-    (ranging from <literal>DEBUG</literal> to <literal>PANIC</literal>) and a primary
+    (ranging from <literal>DEBUG</literal> to <literal>PANIC</literal>, defined
+    in <filename>src/include/utils/elog.h</filename>) and a primary
     message text.  In addition there are optional elements, the most
     common of which is an error identifier code that follows the SQL spec's
     SQLSTATE conventions.