fix(admin): use an absolute path in the file logging example

[chiliec]

☑️ Resolves

• Fix nextcloud.log fails "check for expected files" while update #13258

The file logging example in admin_manual/configuration_server/logging_configuration.rst set
"logfile" => "nextcloud.log". A bare relative path is not resolved against datadirectory
(LogFactory::buildLogFile only applies the datadirectory default when the key is absent), so the log
lands in the installation directory and the updater's "check for expected files" fails, as reported in
the issue and confirmed by @joshtrichards.

Changes:

• example now uses an absolute path (/var/log/nextcloud/nextcloud.log)
• short note that logfile is optional, defaults to nextcloud.log in datadirectory, and should be an
  absolute path when set

🖼️ Screenshots

Rendered text of the changed paragraph (from a local sphinx-build -b html of admin_manual):

The logfile parameter is optional. If it is omitted, the log is written to nextcloud.log in the
datadirectory. When setting it, use an absolute path: a relative path such as "nextcloud.log" is resolved
against the working directory of the PHP process (usually the Nextcloud installation directory), and a log file
placed there makes the updater's check for expected files fail.

✅ Checklist

• I have built the documentation locally and reviewed the output (sphinx-build -q -b html on admin_manual: exit 0, no warnings)
• Screenshots are included for visual changes (text-only change; rendered paragraph quoted above)
• I have not moved or renamed pages (or added a redirect if I did)
• I have run codespell or similar and addressed any spelling issues (sphinx-lint and codespell on the file: clean)