docs: admin-guide: add IGNORE_DIRS example for cscope

docs: admin-guide: add IGNORE_DIRS example for cscope

The workload tracing guide shows how to build a cscope database by
running cscope command directly. The kernel build system also provides
a cscope target, which supports IGNORE_DIRS for excluding directories
from the generated database.

Mention make cscope and show how to exclude Documentation/ as an example.

Signed-off-by: Cheng-Han Wu <hank20010209@gmail.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260503101429.254394-5-hank20010209@gmail.com>