docs: 150题添加注释

problems/0150.逆波兰表达式求值.md

@@ -200,6 +200,7 @@ class Solution(object):
             else:
                 op2 = stack.pop()
                 op1 = stack.pop()
+		# 由题意"The division always truncates toward zero"，所以使用int()可以天然取整
                 stack.append(str(int(eval(op1 + token + op2))))
         return int(stack.pop())
 ```