[llvm] [BOLT][NFC] Clarify requirement of PLTCall pass (PR #164280)

[Gergely Bálint via llvm-commits]

bgergely0 wrote:

Not the most important PR, but it took me some time to understand why PLTCall works on a non-znow-linked input I tried, so I thought it adds some value.

https://github.com/llvm/llvm-project/pull/164280