Update #4455, Regenerate all commands' docs (#4557)

docs/commands/def.md

@@ -16,3 +16,15 @@ Define a custom command
  -  `params`: parameters
  -  `block`: body of the definition
 
+## Examples
+
+Define a command and run it
+```shell
+> def say-hi [] { echo 'hi' }; say-hi
+```
+
+Define a command and run it with parameter(s)
+```shell
+> def say-sth [sth: string] { echo $sth }; say-sth hi
+```
+