doc: Extend documentation of PG_TEST_EXTRA

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 f47ed79cc8 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