Complement documentation about maintenance (data, ref. metadatasets and partial languages for structures)

[dosse]

I'm sorry for the late reporting. According to changes decided in TWG meeting in May 2023:

SDMX standard - Review actions & missing values V6.docx

• data are maintained using their Action property
• ref. metadatasets are maintained using HTTP verbs
• ref. metadatasets retrieved with only partial languages are marked with a new Boolean artefact property isPartialLanguage
• likewise, structure artefacts retrieved with only partial languages are marked with a new Boolean artefact property isPartialLanguage

I can prepare a PR for a next release.