chore: add comment for KafkaSplit's start_offset (#18766)

Signed-off-by: xxchan <xxchan22f@gmail.com>

src/connector/src/source/kafka/split.rs

@@ -22,6 +22,14 @@ use crate::source::{SplitId, SplitMetaData};
 pub struct KafkaSplit {
     pub(crate) topic: String,
     pub(crate) partition: i32,
+    /// Note: currently the start offset is **exclusive**. We need to `+1` to create the reader.
+    /// Possible values are:
+    /// - `Earliest`: `low_watermark` - 1
+    /// - `Latest`: `high_watermark` - 1
+    /// - `Timestamp`: `offset_for_timestamp` - 1
+    /// - `last_seen_offset`
+    ///
+    /// A better approach would be to make it **inclusive**. <https://github.com/risingwavelabs/risingwave/pull/16257>
     pub(crate) start_offset: Option<i64>,
     pub(crate) stop_offset: Option<i64>,
     #[serde(skip)]