[clang] [clang][docs] Improve "Obtaining Clang" section (PR #71313)

[Robin Caloudis via cfe-commits]

robincaloudis wrote:

> Though please make sure to update the PR message (which will become the commit message) before merging, as it's no longer accurate

Thanks. Description is updated 👍 

https://github.com/llvm/llvm-project/pull/71313