message: Add some hints to the docs for `cfg()` targets
diff: @@ -461,6 +461,19 @@ Like with Rust, the syntax here supports the `not`, `any`, and `all` operators
  to combine various cfg name/value pairs. Note that the `cfg` syntax has only
  been available since Cargo 0.9.0 (Rust 1.8.0).
  +If you want to know which cfg targets are available on your platform, run
  +`rustc --print=cfg` from the command line. If you want to know which `cfg`
  +targets are available for another platform, such as 64-bit Windows,
  +run `rustc --print=cfg --target=x86_64-pc-windows-msvc`.
  +If you want to know which cfg targets are used by the release profile,
  +add the `-O` parameter.
  +
  +This same set of values is used throughout the build for all crates in your project,
  +so, unlike in your Rust source code,
  +you cannot use `[target.'cfg(feature = "my_crate")'.dependencies]` to add
  +dependencies based on optional crate features.
  +Use [the `[features]` section](manifest.md#the-features-section) instead.
  +
  In addition to `#[cfg]` syntax, Cargo also supports listing out the full target
  the dependencies would apply to:
  