NIFI-15810 Document missing RecordPath functions

[awelless]

Summary

NIFI-15810

In addition to the changes, there are some functions which are present in the repository docs, but are absent in the public docs. E.g. Math Functions, like toNumber. That's a reference to v1 docs.

Tracking

Please complete the following tracking steps prior to pull request creation.

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
• Pull request contains commits signed with a registered key indicating Verified status

Pull Request Formatting

• Pull Request based on current revision of the main branch
• Pull Request refers to a feature branch with one commit containing changes

Verification

Please indicate the verification steps performed prior to pull request creation.

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. Verified with

./mvnw -q -pl nifi-docs package -DskipTests && open nifi-docs/target/generated-docs/record-path-guide.html