Update 4.fetch.md (#811)

* Update 4.fetch.md

* Update 4.fetch.md

docs-2.0/3.ngql-guide/7.general-query-statements/4.fetch.md

@@ -1,5 +1,7 @@
 # FETCH
 
+<!-- This topic needs to be modified after compulsory use of YIELD in 3.0. Refer to https://confluence.nebula-graph.io/pages/viewpage.action?pageId=10723963 -->
+
 The `FETCH` statement retrieves the properties of the specified vertices or edges.
 
 ## OpenCypher Compatibility
@@ -13,15 +15,16 @@ This topic applies to native nGQL only.
 ```ngql
 FETCH PROP ON {<tag_name>[, tag_name ...] | *} 
 <vid> [, vid ...] 
-[YIELD <output>]
+[YIELD <return_list> [AS <alias>]]; 
 ```
 
 |Parameter|Description|
 |:---|:---|
 |`tag_name`|The name of the tag.|
 |`*`|Represents all the tags in the current graph space.|
 |`vid`|The vertex ID.|
-|`output`|Specifies the information to be returned. For more information, see [`YIELD`](../8.clauses-and-options/yield.md). If there is no `YIELD` clause, `FETCH` returns all the matched information.|
+|`YIELD`|Define the output to be returned. The defined properties and `VertexID` are returned. For details, see [`YIELD`](../8.clauses-and-options/yield.md). If there is no `YIELD` clause, `vertices_` is returned by default, which contains all the information about the vertex. |
+|`AS`| Set an alias.|
 
 ### Fetch vertex properties by one tag
 
@@ -42,9 +45,9 @@ Use a `YIELD` clause to specify the properties to be returned.
 
 ```ngql
 nebula> FETCH PROP ON player "player100" \
- YIELD player.name;
+ YIELD player.name AS name;
 +-------------+--------------+
-| VertexID | player.name |
+| VertexID | name  |
 +-------------+--------------+
 | "player100" | "Tim Duncan" |
 +-------------+--------------+
@@ -132,7 +135,7 @@ FETCH PROP ON <edge_type> <src_vid> -> <dst_vid>[@<rank>] [, <src_vid> -> <dst_v
 |`src_vid`|The VID of the source vertex. It specifies the start of an edge.|
 |`dst_vid`|The VID of the destination vertex. It specifies the end of an edge.|
 |`rank`|The rank of the edge. It is optional and defaults to `0`. It distinguishes an edge from other edges with the same edge type, source vertex, destination vertex, and rank.|
-|`output`|Specifies the information to be returned. For more information, see [`YIELD`](../8.clauses-and-options/yield.md). If there is no `YIELD` clause, `FETCH` returns all the matched information.|
+|`YIELD`|Define the output to be returned. The defined properties, `SrcVertexID`, `DstVertexID`, and `rank` are returned. For details, see [`YIELD`](../8.clauses-and-options/yield.md). If there is no `YIELD` clause, `edges_` is returned by default, which contains all the information about the edge. |
 
 ### Fetch all properties of an edge