>>9277989
Okay. I made some visual aid because desu it is impossible to explain this because I've always had an intuitive understanding of this.
When you do polar coordinates the "r" is simply a stick that is swiping through the plane. With the theta argument you tell the stick from where to where it has to swipe.
In here you see that your circle is completely in the right region. So if you can see that if you want r to "swipe" just that region, you need to tell it to go from theta=0 to theta= pi/2. If it goes all the way to pi then r will also swipe the left part of the plane, which is not in region.