I just find in ABAP help documentation there are also similar syntax graph to illustrate the grammar of each keyword.
Just open one ABAP report, select any keyword and press F1, and you can find “ABAP syntax diagrams”.
Double click on it and choose one keyword like “APPEND” in the right area:
Then the syntax diagram is opened. Ckick small “+” icon to drill down.
Click the “?” icon to get the meaning of each legend used in the graph.