[clang] [Docs] Document freestanding requirements (PR #132232)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 commented:

I think this should be more specific on what `-ffreestanding` actually does. Namely, removed implicit search paths, link libraries, passes `-fno-builtin` and lets `int main` function as a normal function.

For the `libc` side, I don't think this makes much of a difference, but it's nice to see the function requirements documented.

https://github.com/llvm/llvm-project/pull/132232