[skip ci]Add document for sealed segment variable and functions (#9364)

Signed-off-by: Nxiaofan-luan <xiaofan.luan@zilliz.com>

docs/design_docs/segcore/segment_sealed.md

 # SegmentSealed
 SegmentSealed has extra interface rather than segment_inferface:
 
-1. `LoadIndex(loadIndexInfo)`: load the index. indexInfo containts
+1. `LoadIndex(loadIndexInfo)`: load the index. indexInfo contains
     1. `FieldId`
     2. `IndexParams`: index paramters in KV structure KV 
     3. `VecIndex`: vector index
@@ -10,4 +10,32 @@ SegmentSealed has extra interface rather than segment_inferface:
 3. `DropIndex(fieldId)`: drop and release exist index of specified field 
 4. `DropFieldData(fieldId)`: drop and release exist data for specified field
 
-Search is executatble as long as all the column involved in the search are loaded.
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+Search is executatble as long as all the column involved in the search are loaded.
+
+# SegmentSealedImpl internal data definition
+1. `row_count_opt_`:
+   1. Fill row count when load the first entity
+   2. All the other column loaded must match the same row count
+3. `xxx_ready_bitset_` `system_ready_count_`
+   1. Used to record whether the corresponding column is loaded. Bitset corresponds to FieldOffset
+   2. Query is executatble If and only if the following conditions are met:
+      1. system_ready_count_ == 2， which means all the system column RowId/Timestamp is loaded
+      2. The scalar columns involved in the query has been loaded
+      3. For the vector columns involved in the query, either the original data or the index is loaded
+4. `scalar_indexings_`: Store scalar index
+
+   1. Use StructuredSortedIndex in knowhere 
+5. `primary_key_index_`: store index for pk column
+   1. Use brand new ScalarIndexBase format
+   2. **Note，The functions here may overlap with scalar indexes. It is recommended to replace scalar index with ScalarIndexBase**
+6. `field_datas_`: store original data
+   1. `aligned_vector<char>` format ganruatee `int/float` data are aligned
+7. `SealedIndexingRecord vecindexs_`: store vector index
+8. `row_ids_/timestamps_`: RowId/Timestamp data
+9. `TimestampIndex`: Index for Timestamp column
+10. `schema`: schema
+
+# SegmentSealedImpl internal function dafinion
+1.Most functions are the implementation of the corresponding functions of segment interface, which will not be repeated here
+2. `update_row_count`: Used to update the row_count field
+3. `mask_with_timestamps`: Use Timestamp column to update search bitmask，used to support Time Travel function
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