add link to sqlite source about payload_overflows()

core/storage/btree.rs

@@ -1494,6 +1494,7 @@ impl BTreeCursor {
         log::debug!("fill_cell_payload(overflow)");
 
         let payload_overflow_threshold_min = self.payload_overflow_threshold_min(page_type);
+        // see e.g. https://github.com/sqlite/sqlite/blob/9591d3fe93936533c8c3b0dc4d025ac999539e11/src/dbstat.c#L371
         let mut space_left = payload_overflow_threshold_min
             + (record_buf.len() - payload_overflow_threshold_min) % (self.usable_space() - 4);
 

core/storage/sqlite3_ondisk.rs

@@ -1274,6 +1274,7 @@ pub fn begin_write_wal_header(io: &Rc<dyn File>, header: &WalHeader) -> Result<(
 /// Checks if payload will overflow a cell based on the maximum allowed size.
 /// It will return the min size that will be stored in that case,
 /// including overflow pointer
+/// see e.g. https://github.com/sqlite/sqlite/blob/9591d3fe93936533c8c3b0dc4d025ac999539e11/src/dbstat.c#L371
 pub fn payload_overflows(
     payload_size: usize,
     payload_overflow_threshold_max: usize,