docs: clarify intermediate certificate creation instructions

Specifically, explain the v3_ca openssl specification.

Discussion: https://postgr.es/m/20200824175653.GA32411@momjian.us

Backpatch-through: 9.5

doc/src/sgml/runtime.sgml

@@ -2193,8 +2193,10 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
    The certificates of <quote>intermediate</quote> certificate authorities
    can also be appended to the file.  Doing this avoids the necessity of
    storing intermediate certificates on clients, assuming the root and
-   intermediate certificates were created with <literal>v3_ca</literal>
-   extensions.  This allows easier expiration of intermediate certificates.
+   intermediate certificates were created with <literal>v3_ca </literal>
+   extensions.  (This sets the certificate's basic constraint of
+   <literal>CA</literal> to <literal>true</literal>.)
+   This allows easier expiration of intermediate certificates.
   </para>
 
   <para>