[llvm][Docs] Make it clear where lit test files live (#124121)
As someone on Discord was understandably confused because the build
directory does contain folder structures that look remarkably like the
source directory.
I used this page to explain it but realised that this must be from when
llvm was a separate repository. So `<user home>/llvm` probably was a
common path.
Now it's in llvm-project. So make that obvious in the instructions.