NIFI-15924 some Admin Guide fixes

[mosermw]

Summary

NIFI-15924

• The nifi.restore.directory incorrectly contains some info meant for nifi.nar.library.directory
• In the Autoloading Custom Processors section, requirements for directory and file permissions could be made more clear.

Tracking

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Issue Tracking

• Apache NiFi Jira issue created

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000
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Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • JDK 25

Licensing

• New dependencies are compatible with the Apache License 2.0 according to the License Policy
• New dependencies are documented in applicable LICENSE and NOTICE files

Documentation

• Documentation formatting appears as expected in rendered files

[exceptionfactory]

Thanks @mosermw! +1 merging