doc: Extend documentation of PG_TEST_EXTRA

Extend the PG_TEST_EXTRA documentation to mention resource intensive
tests as well. The previous wording only mentioned special software
and security in the main paragraph, with resource usage listed on one
of the tests in the list.

Backpatch to v15 where f47ed79 added wal_consistenct_checking as
a PG_TEST_EXTRA target.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAN55FZ0OthTuBdiNkaX2BvxuHdK4Y1MVEb8_uEuD1yHMPmT9Og@mail.gmail.com
Backpatch-through: 15

Author: danielgustafsson

doc/src/sgml/regress.sgml

@@ -253,11 +253,11 @@ make check-world -j8 >/dev/null
 
   <para>
    Some test suites are not run by default, either because they are not secure
-   to run on a multiuser system or because they require special software.  You
-   can decide which test suites to run additionally by setting the
-   <command>make</command> or environment variable
-   <varname>PG_TEST_EXTRA</varname> to a whitespace-separated list, for
-   example:
+   to run on a multiuser system, because they require special software or
+   because they are resource intensive.  You can decide which test suites to
+   run additionally by setting the <command>make</command> or environment
+   variable <varname>PG_TEST_EXTRA</varname> to a whitespace-separated list,
+   for example:
 <programlisting>
 make check-world PG_TEST_EXTRA='kerberos ldap ssl load_balance'
 </programlisting>