add comment to clarify default gas value

Author: emc415

vyper/builtins/functions.py

@@ -1246,6 +1246,7 @@ class Send(BuiltinFunction):
 
     _id = "send"
     _inputs = [("to", AddressT()), ("value", UINT256_T)]
+    # default gas stipend is 0
     _kwargs = {"gas": KwargSettings(UINT256_T, 0)}
     _return_type = None
 
@@ -1256,6 +1257,7 @@ def build_IR(self, expr, args, kwargs, context):
         context.check_is_not_constant("send ether", expr)
         return IRnode.from_list(["assert", ["call", gas, to, value, 0, 0, 0, 0]])
 
+
 class SelfDestruct(BuiltinFunction):
 
     _id = "selfdestruct"