Document missing env vars for 8.8

[zalanszak]

The following env vars are missing in the 8.8 property reference/docs:

CAMUNDA_OPERATE_OPENSEARCH_SOCKETTIMEOUT, CAMUNDA_TASKLIST_OPENSEARCH_SOCKETTIMEOUT

Please document them.

I would also appreciate an explanation on what the differences are between these two and
CAMUNDA_DATA_BACKUP_SNAPSHOTTIMEOUT (the documented env var)

Related ticket: https://jira.camunda.com/browse/SUPPORT-31358

[mesellings]

Hi @zalanszak these weren't in 8.8, but only in 8.9 according to the master unified config spreadsheet - so you can find them in the 8.9. docs but not in 8.8 - this is correct isn't it @buccarel?