#6083 update examples in File part of Native API: 'Editing Variable Level Metadata' section.

[pkiraly]

What this PR does / why we need it: It improves the Native API documentation by providing full curl examples for a File regarding call in 'Editing Variable Level Metadata' section

Which issue(s) this PR closes: none of them. It is an additional step for solving #6083

Special notes for your reviewer: This PR continue previous pull requests which handles examples regarding dataverses/datasets/files (e.g. #6600). This one is about reformatting a file API call. It was not clear for me if this call wirks both with file ID and persistent ID, so I just improved the example for the file id, and did not add example for the persistent ID.

Suggestions on how to test this:

cd doc/sphinx-guides
make clean && make html
firefox build/html/api/native-api.html

Check 'Editing Variable Level Metadata' section

Does this PR introduce a user interface change?: no

Is there a release notes update needed for this change?: no

Additional documentation: no

[coveralls]

Coverage remained the same at 19.471% when pulling 3e791d6 on pkiraly:6083-documentation-update-native-API-examples-editing-variable-level-metadata into afcd2aa on IQSS:develop.

[mheppler]

👍 Looks good.

[pkiraly]

This PR was developed by Göttingen eResearch Alliance, Germany, and funded by SSHOC, "Social Sciences and Humanities Open Cloud". SSHOC has received funding from the European Union’s Horizon 2020 project call H2020-INFRAEOSC-04-2018, grant agreement #823782.