add schema function (#968)

* add schema function

* Update 4.schema.md

docs-2.0/3.ngql-guide/6.functions-and-expressions/4.schema.md

@@ -9,15 +9,24 @@ Nebula Graph supports the following schema functions.
     - The following functions are available in both the WHERE and YIELD clauses in GO statements.
     - The following functions are only available in the YIELD clauses in LOOKUP and YIELD statements.
 
-| Function               | Description                                                                                             |
-|------------------------+---------------------------------------------------------------------------------------------------------|
+| Function               | Description                    |
+|:-----------------------|:---------------------------------------|
 | id(vertex)             | Returns the ID of a vertex. The data type of the result is the same as the vertex ID.                   |
 | map properties(vertex) | Returns the properties of a vertex.                                                                     |
 | map properties(edge)   | Returns the properties of an edge.                                                                      |
 | string type(edge)      | Returns the edge type of an edge.                                                                       |
 | src(edge)              | Returns the source vertex ID of an edge. The data type of the result is the same as the vertex ID.      |
 | dst(edge)              | Returns the destination vertex ID of an edge. The data type of the result is the same as the vertex ID. |
-| int rank(edge)         | Returns the rank value of an edge.                                                                      |
+| int rank(edge)         | Returns the rank value of an edge.   |
+|vertex | Returns the information of vertices, including VIDs, tags, properties, and values.|
+|edge | Returns the information of edges, including edge types, source vertices, destination vertices, ranks, properties, and values.|
+|vertices | Returns the information of vertices in a subgraph. For more information, see [GET SUBGRAPH](../16.subgraph-and-path/1.get-subgraph.md)。|
+|edges | Returns the information of edges in a subgraph. For more information, see [GET SUBGRAPH](../16.subgraph-and-path/1.get-subgraph.md)。|
+|path  | Returns the information of a path. For more information, see [FIND PATH](../16.subgraph-and-path/2.find-path.md)。|
+
+!!! note
+
+    Since vertex, edge, vertices, edges, and path are keywords, you need to use `AS <alias>` to set the alias, such as `GO FROM "player100" OVER follow YIELD edge AS e;`.
 
 ## For statements compatible with openCypher