Add some hints to the docs for cfg() targets

https://users.rust-lang.org/t/what-cfg-expressions-are-expected-to-work-in-cargo-toml-quoted-targets/28662

Author: notriddle

src/doc/src/reference/specifying-dependencies.md

@@ -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: