Documentation: power: Correct a mistaken configuration option

Documentation: power: Correct a mistaken configuration option

Somehow CONFIG_PSTORE_FIRMWARE ended up in this document when I intended
it to be CONFIG_CHROMEOS_PSTORE.  Correct the configuration option and
make it clear that not all options are required.

Fixes: b1f02f005a2e ("Documentation: power: Add document on debugging shutdown hangs")
Reported-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
[ rjw: Fixes: tag ]
Link: https://patch.msgid.link/20251106142524.3841343-1-superm1@kernel.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>