message: update build dependencies profiles documentation
  This describes the new defaults for build-overrides, and how to make sure backtraces have the usual debug info, when needed.
diff: @@ -298,20 +298,29 @@ The `bench` profile inherits the settings from the [`release`](#release) profile
  #### Build Dependencies
  -All profiles, by default, do not optimize build dependencies (build scripts,
  -proc macros, and their dependencies). The default settings for build overrides
  -are:
  +To compile quickly, all profiles, by default, do not optimize build
  +dependencies (build scripts, proc macros, and their dependencies), and avoid
  +computing debug info when a build dependency is not used as a runtime
  +dependency. The default settings for build overrides are:
  ```toml
  [profile.dev.build-override]
  opt-level = 0
  codegen-units = 256
  +debug = false # when possible
  [profile.release.build-override]
  opt-level = 0
  codegen-units = 256
  ```
  +However, if errors occur while running build dependencies, turning full debug
  +info on will improve backtraces and debuggability when needed:
  +
  +```toml
  +debug = true
  +```
  +
  Build dependencies otherwise inherit settings from the active profile in use, as
  described in [Profile selection](#profile-selection).
  