various minor improvements to nim generator (#3883)

Author: wing328

docs/generators.md

@@ -38,7 +38,7 @@ The following generators are available:
 * [jmeter](generators/jmeter)  
 * [kotlin](generators/kotlin)  
 * [lua](generators/lua)  
-* [nim](generators/nim)  
+* [nim (beta)](generators/nim)  
 * [objc](generators/objc)  
 * [ocaml](generators/ocaml)  
 * [perl](generators/perl)