Add missing clean room constraints

flashloaders/cleanroom.md

@@ -13,7 +13,7 @@ English version can be found below.
 `r1`: 拷贝终点起始地址
 `r2`: 拷贝word（4字节）数(存在例外)
 
-程序功能：将数据从源点拷贝到终点，在拷贝完毕后触发断点以结束执行
+程序功能：将数据从源点拷贝到终点，在拷贝完毕后触发断点以结束执行，结束时`r2`值需清零表明传输完毕。
 
 限制：不可使用栈，可自由使用的临时寄存器为`R3`到`R12`。`R13`为`sp`（stack pointer），`R14`为lr（一般用于储存跳转地址），`R15`为`pc`（program counter）。
 
@@ -132,7 +132,7 @@ All parameters would be passed over registers
 
 **What the program is expected to do**:
 
-Copy data from source to destination, after which trigger a breakpint to exit.
+Copy data from source to destination, after which trigger a breakpint to exit. Before exit, `r2` must be cleared to zero to indicate that the copy is done.
 
 **Limitation**: No stack operations are permitted. Registers ranging from `r3` to `r12` are free to use. Note that `r13` is `sp`(stack pointer), `r14` is `lr`(commonly used to store jump address), `r15` is `pc`(program counter).