message: Update default for codegen-units.
diff: @@ -186,8 +186,8 @@ possibly reducing compile time, but may produce slower code.
  This option takes an integer greater than 0.
  -This option is ignored if [incremental](#incremental) is enabled, in which
  -case `rustc` uses an internal heuristic to split the crate.
  +The default is 256 for [incremental](#incremental) builds, and 16 for
  +non-incremental builds.
  [`-C codegen-units` flag]: ../../rustc/codegen-options/index.html#codegen-units
  @@ -217,7 +217,7 @@ overflow-checks = true
  lto = false
  panic = 'unwind'
  incremental = true
  -codegen-units = 16 # Note: ignored because `incremental` is enabled.
  +codegen-units = 256
  rpath = false
  ```
  @@ -258,7 +258,7 @@ overflow-checks = true
  lto = false
  panic = 'unwind' # This setting is always ignored.
  incremental = true
  -codegen-units = 16 # Note: ignored because `incremental` is enabled.
  +codegen-units = 256
  rpath = false
  ```
  