Add docs for closures.

Cargo.toml

@@ -23,7 +23,7 @@ smallvec = { version = "1.4.1", default-features = false }
 [features]
 #default = ["unchecked", "sync", "no_optimize", "no_float", "only_i32", "no_index", "no_object", "no_function", "no_module"]
 default = []
-plugins = []
+plugins = []        # custom plugins support
 unchecked = []      # unchecked arithmetic
 sync = []           # restrict to only types that implement Send + Sync
 no_optimize = []    # no script optimizer