docs: explain field

Signed-off-by: discord9 <discord9@163.com>

src/common/meta/src/rpc/ddl.rs

@@ -1432,8 +1432,10 @@ pub struct QueryContext {
     pub timezone: String,
     pub extensions: HashMap<String, String>,
     pub channel: u8,
+    /// Maps region id -> snapshot upper bound sequence for that region.
     #[serde(default)]
     pub snapshot_seqs: HashMap<u64, u64>,
+    /// Maps region id -> minimal SST sequence allowed for that region.
     #[serde(default)]
     pub sst_min_sequences: HashMap<u64, u64>,
 }

src/query/src/options.rs

@@ -60,9 +60,13 @@ pub enum FlowIncrementalMode {
 
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 pub struct FlowQueryExtensions {
+    /// Maps region id -> lower exclusive sequence bound for incremental reads.
     pub incremental_after_seqs: Option<HashMap<u64, u64>>,
+    /// Incremental read mode requested by the caller.
     pub incremental_mode: Option<FlowIncrementalMode>,
+    /// Whether the caller expects per-region watermark metadata in terminal metrics.
     pub return_region_seq: bool,
+    /// Optional sink table id used to distinguish source scans from sink reads.
     pub sink_table_id: Option<TableId>,
 }