[DOC OCP] "Changing a topic" section does not include info on changing replicas via TO

Based on feedback from the field, we need to document that changing replicas using the Topic Operator is not currently supported. This should be documented in the existing "Creating a topic" procedure. The procedure includes this note:

NOTE: Changing topic names using the KafkaTopic.spec.topicName variable and decreasing partition size using the KafkaTopic.spec.partitions variable is not supported by Kafka.

Nothing is mentioned about changing replicas here.