doc: Clarify the errhint example

The errhint example wasn't using the mandated style for errhint
which was needlessly confusing.  Fix by rewriting it using the
mandated style.

Reported-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Jonathan S. Katz <jkatz@postgresql.org>
Discussion: https://postgr.es/m/F02F4B4F-7B7E-406A-B25A-FF2AA300AB9A@yesql.se

doc/src/sgml/sources.sgml

@@ -470,7 +470,7 @@ IpcMemoryCreate: shmget(key=%d, size=%u, 0%o) failed: %m
 <programlisting>
 Primary:    could not create shared memory segment: %m
 Detail:     Failed syscall was shmget(key=%d, size=%u, 0%o).
-Hint:       the addendum
+Hint:       The addendum, written as a complete sentence.
 </programlisting>
    </para>