-Z split-dwarf-kind={single,split} is undocumented in the unstable book

[jieyouxu]

Implementation history:

• In rustc: Stabilize -Z run-dsymutil as -C split-debuginfo #79570:
  • -Z split-dwarf=single became -C split-debuginfo=packed
  • -Z split-dwarf=split became -C split-debuginfo=unpacked
• However, as @davidtwco noted in 08ed338 (cg: split dwarf for crate dependencies #89819):

  In rustc: Stabilize -Zrun-dsymutil as -Csplit-debuginfo #79570, -Z split-dwarf-kind={none,single,split} was replaced by -C split-debuginfo={off,packed,unpacked}. -C split-debuginfo's packed and unpacked aren't exact parallels to single and split, respectively.

• As such, cg: split dwarf for crate dependencies #89819 introduced -Z split-dwarf-kind={single,split} to fill in the gap.

This unstable compiler flag doesn't seem to have an entry in the unstable book, I only discovered them while looking at tests/run-make/split-debuginfo. It would be nice to document its behavior and also the meaning of its values.