Clarify a comment.

compiler/rustc_middle/src/mir/terminator.rs

@@ -86,7 +86,7 @@ pub fn target_for_value(&self, value: u128) -> BasicBlock {
         self.iter().find_map(|(v, t)| (v == value).then_some(t)).unwrap_or_else(|| self.otherwise())
     }
 
-    /// Adds a new target to the switch. But You cannot add an already present value.
+    /// Adds a new target to the switch. Panics if you add an already present value.
     #[inline]
     pub fn add_target(&mut self, value: u128, bb: BasicBlock) {
         let value = Pu128(value);