docs(15.9): use the snake_case field names of the admin API

[marevol]

Summary

The 15.9 admin REST API pages documented the JSON fields in camelCase. Fess maps admin API bodies with CAMEL_TO_LOWER_SNAKE, so the real request and response fields are snake_case. Sending camelCase makes only the multi-word fields fail as "required", e.g. scriptType instead of script_type.

This PR changes every documented JSON key in the request/response examples, field tables and curl examples to the name the API actually uses, in all 7 locales and in 15.9 only. Single-word names are unchanged. The names were taken from each ApiAdmin*Action and its Body/Form classes.

• Some names deliberately stay camelCase, because they are not JSON body fields:
  • map keys (user/group attributes, log/storage lastModified/hashCode);
  • multipart upload fields (e.g. badWordFile).
• Fess leaves null values out of responses, so the pages no longer say that ldap_admin_security_credentials and job_log_id come back as null.
• Two heading underlines that were too short (ja and de boostdoc) are fixed.

Verification

• tools/check_headings.py passes for all 7 locales.
• docutils reports no new messages for the 161 changed files.
• A Sphinx build shows the same warnings before and after.
• The field names match across all locales.