[llvm] [Docs] Clarify that `reassoc` isn't just for reassociation (PR #133168)

[Chris B via llvm-commits]

llvm-beanz wrote:

As a note, the language I'm proposing here is based on the language from the LLVM 5 language reference:

> Fast - Allow algebraically equivalent transformations that may dramatically change results in floating point (e.g. reassociate). This flag implies all the others.

(see: https://releases.llvm.org/5.0.0/docs/LangRef.html#fast-math-flags)

The modern `reassoc` flag is really inherited from the unsafe algebra behaviors that came with the `fast` flag from older LLVM.

https://github.com/llvm/llvm-project/pull/133168