Add a tip for VID modification (#1157)

docs-2.0/1.introduction/3.vid.md

@@ -44,9 +44,11 @@ VIDs can be generated via applications. Here are some tips:
 
 - If you generate int64 VID via hash, the probability of collision is about 1/10 when there are 1 billion vertices. The number of edges has no concern with the probability of collision.
 
-## Define and modify the data type of VIDs
+## Define and modify a VID and its data type
 
-The data type of VIDs must be defined when you [create the graph space](../3.ngql-guide/9.space-statements/1.create-space.md). Once defined, it cannot be modified.
+The data type of a VID must be defined when you [create the graph space](../3.ngql-guide/9.space-statements/1.create-space.md). Once defined, it cannot be modified.
+
+A VID is set when you [insert a vertex](../3.ngql-guide/12.vertex-statements/1.insert-vertex.md) and cannot be modified. 
 
 ## Query `start vid` and global scan