[lld] [lld][docs] Document two linker-script related options for lld ELF (PR #166313)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

LGTM.

I would probably have stated that `-T` is the short alias for `--script`, and `-dT` is the short alias for `--default-script`, but the contents here is clear enough, so I don't think you need to update the text unless you want to.

https://github.com/llvm/llvm-project/pull/166313