[llvm] [CodeGen] Add comment to MachineBasicBlock::instrs method. (PR #113868)

[Afanasyev Ivan via llvm-commits]

ivafanas wrote:

@chandlerc @dsandersllvm 

Could you please review the PR?

I should have triggered Evan Cheng as maintainer of `MachineBasicBlock.h`, but can't find Evan's github account in `Maintainers.md`.

https://github.com/llvm/llvm-project/pull/113868