[lld] [LLD] [Docs] explain --unresolved-symbols options (PR #165937)

[Peter Collingbourne via llvm-commits]

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

> Tested with `groff -t -e -mandoc -Tascii lld/docs/ld.lld.1 | less`

(Easier way: `man lld/docs/ld.lld.1`)

https://github.com/llvm/llvm-project/pull/165937