docs: fix 'make htmldocs' warning in perf

mainline inclusion
from mainline-v5.17-rc1
commit 0d6356d6
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/IAQG34

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0d6356d6cdd0afeb546f4e8ca653748a8ec3ba26



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Fix following 'make htmldocs' warnings:
./Documentation/admin-guide/perf/hisi-pcie-pmu.rst: WARNING:
document isn't included in any toctree

Fixes: c8602008 ("docs: perf: Add description for HiSilicon PCIe PMU driver")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20220228031700.1669086-1-wanjiabing@vivo.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>