Change response format of update_documents

[wanliAlex]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) bug fix

• What is the current behavior? (You can also link to an open issue here) we return indexName, preprocessingTime

• What is the new behavior (if this is a feature change)? we return index_time, processingTimeMs to be consistent with add_documents

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) no

• Have unit tests been run against this PR? (Has there also been any additional testing?) no

• Related Python client changes (link commit/PR here) no

• Related documentation changes (link commit/PR here) no

• Other information: no

• Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines

• [x] Tests for the changes have been added (for bug fixes/features)

• [x] Docs have been added / updated (for bug fixes / features)