docs: clarify +1 offset semantics for store_offset

[funcpp]

commit and commit_message already document the +1 offset convention, but the store_offset* methods don't. They also don't agree with each other: store_offset stores offset + 1, while store_offsets stores offsets as is. That asymmetry is easy to miss without reading librdkafka's docs.

The wording is taken from librdkafka's own docs:

• rd_kafka_offset_store: Store offset `offset` + 1 for topic `rkt` partition `partition`.
• rd_kafka_offset_store_message: Store offset +1 for the consumed message.
• rd_kafka_offsets_store: @remark The `.offset` field is stored as is, it will NOT be + 1.