chore(postgresflex): add more descriptions

[What-is-water93]

Description

Added descriptions for the replicas and the backup_schedule parameters, the content is derived from the errors you receive when using the wrong values

│ Calling API: 400 Bad Request, status code 400, Body: {"message":"failed to validate
│ body","code":400,"type":"Validation","fields":{"backupSchedule":["the backupSchedule field is
│ not valid: invalid backup schedule syntax. Must be a valid cron expression using numeric
│ minute and hour values, e.g: '0 2 * * *'"],"replicas":["the replicas field is not valid:
│ postgres instance replicas not supported, supported replicas are '1' and '3'"]}}

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)