Improve Bigtable operator documentation

[sawantbandita]

Added clearer explanations for Bigtable operators and removed duplicate sentences for better readability.
Closes #61269

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[boring-cyborg]

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[shahar1]

Small nitpicks, overall LGTM!

[KamranImaaz]

BigtableTableReplicationCompletedSensor section is a sensor specific documentation not a part of operator documentation. This section should be moved to Sensor specific documentation.

[boring-cyborg]

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