[llvm] [SandboxIR][NFC] Add some comments (PR #99359)

[Arthur Eubanks via llvm-commits]

================
@@ -415,6 +423,7 @@ class Constant : public sandboxir::User {
 };
 
 /// The BasicBlock::iterator.
+/// It returns a sandboxir::BasicBlock & when derereferenced.
----------------
aeubanks wrote:

fix wording to be consistent with above wording like `OperandUseIterator`

https://github.com/llvm/llvm-project/pull/99359