message: Update split-debuginfo docs around the default.
diff: @@ -83,12 +83,12 @@ controls whether debug information, if generated, is either placed in the
  executable itself or adjacent to it.
  This option is a string and acceptable values are the same as those the
  -[compiler accepts][`-C split-debuginfo` flag]. This option is currently not
  -passed by default on platforms other than macOS, where it defaults to
  -`unpacked`. The default may change in the future for platforms using DWARF
  -debugging information and ELF executables to `unpacked` as well once it is
  -stabilized in the compiler.
  +[compiler accepts][`-C split-debuginfo` flag]. See that documentation for the
  +default behavior, which is platform-specific. Some options are only available
  +on the [nightly channel]. The default may change in the future once more
  +testing has been performed, and support for DWARF is stabilized.
  +[nightly channel]: ../../book/appendix-07-nightly-rust.html
  [`-C split-debuginfo` flag]: ../../rustc/codegen-options/index.html#split-debuginfo
  #### debug-assertions
  @@ -230,6 +230,7 @@ The default settings for the `dev` profile are:
  [profile.dev]
  opt-level = 0
  debug = true
  +split-debuginfo = '...' # Platform-specific.
  debug-assertions = true
  overflow-checks = true
  lto = false
  @@ -251,6 +252,7 @@ The default settings for the `release` profile are:
  [profile.release]
  opt-level = 3
  debug = false
  +split-debuginfo = '...' # Platform-specific.
  debug-assertions = false
  overflow-checks = false
  lto = false
  @@ -271,6 +273,7 @@ The default settings for the `test` profile are:
  [profile.test]
  opt-level = 0
  debug = 2
  +split-debuginfo = '...' # Platform-specific.
  debug-assertions = true
  overflow-checks = true
  lto = false
  @@ -291,6 +294,7 @@ The default settings for the `bench` profile are:
  [profile.bench]
  opt-level = 3
  debug = false
  +split-debuginfo = '...' # Platform-specific.
  debug-assertions = false
  overflow-checks = false
  lto = false
  