[Documentation] Document usecases for multi-term aggregation in bucket level monitor

[rishabhmaurya]

Is your feature request related to a problem?
With opensearch-project/OpenSearch#1629 we will be supporting multi-term aggregation. This is helpful when dynamic buckets needs to be create with multiple term sources and ordering is required among them. Composite aggregations serve the same purpose but doesn't supports ordering of buckets.

What solution would you like?
Document usecases.

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
Add any other context or screenshots about the feature request here.