message: Document that CARGO_PKG_ are availble to build.rs
diff: @@ -330,6 +330,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
  changed by editing `.cargo/config.toml`; see the documentation
  about [cargo configuration][cargo-config] for more
  information.
  +* `CARGO_PKG_<var>` - The package information variables, with the same names and values as are [provided during crate building][variables set for crates].
  [unix-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows
  [windows-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows
  @@ -346,6 +347,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
  [jobserver]: https://www.gnu.org/software/make/manual/html_node/Job-Slots.html
  [cargo-config]: config.md
  [Target Triple]: ../appendix/glossary.md#target
  +[variables set for crates]: #environment-variables-cargo-sets-for-crates
  ### Environment variables Cargo sets for 3rd party subcommands
  