[QUESTION] Why tristate look like bool?

[Les Boys]

First, I wrote a minimal Kconfig:
```
config TRISTATE_TEST
    tristate "Test"
```
Then, I ran:
```
./conf --oldaskconfig test.lbConfig
```
Finally, it showed:
```
Test (TRISTATE_TEST) [N/y] (NEW): 
```
This is not excepted, it should shows:
```
Test (TRISTATE_TEST) [N/y/m] (NEW): 
```
I am confused about this problem, no document talks about it, please explain to me, thanks.

-- Les Boys