CBG-4403: update rev cache size api docs for size stat

[gregns1]

CBG-4403

• We recently added a size stat for rev cache, we need to add a disclaimer to rev cache sizing on api docs to show that that the defined size in the api could be exceeded by 10%.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/000/

[github-actions]

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