doc/codegen: update entc command output (#881)

doc/md/code-gen.md

@@ -88,10 +88,11 @@ Examples:
   entc generate github.com/a8m/x
 
 Flags:
+      --feature strings       extend codegen with additional features
       --header string         override codegen header
   -h, --help                  help for generate
       --idtype [int string]   type of the id field (default int)
-      --storage strings       list of storage drivers to support (default [sql])
+      --storage string        storage driver to support in codegen (default "sql")
       --target string         target directory for codegen
       --template strings      external templates to execute
 ```
@@ -228,4 +229,4 @@ func EnsureStructTag(name string) gen.Hook {
 		})
 	}
 }
-```
+```