Fix llvm-strip --only-keep-debug documentation for ELF

The functionality (and llvm-objcopy's corresponding documentation) was
added in
https://github.com/llvm/llvm-project/commit/5ad0103d8a04cb066dfae4fc20b0dfcd9413f4d4.
It looks like the llvm-strip docs to match were missed.

Reviewed by: gbreynoo

Differential Revision: https://reviews.llvm.org/D121902