message: extra-targets -> targets and add documentation
diff: @@ -162,10 +162,16 @@ no-default-features = true
  # - i686-pc-windows-msvc
  default-target = "x86_64-unknown-linux-gnu"
  -# Targets to build in addition to `default-target` (default: all tier 1 targets)
  +# Targets to build (default: all tier 1 targets)
  +#
  # Same available targets as `default-target`.
  # Set this to `[]` to only build the default target.
  -extra-targets = [ "x86_64-apple-darwin", "x86_64-pc-windows-msvc" ]
  +#
  +# If `default-target` is unset, the first element of `targets` is treated as the default target.
  +# Otherwise, these `targets` are built in addition to the default target.
  +# If both `default-target` and `targets` are unset,
  +# all tier-one targets will be built and `x86_64-unknown-linux-gnu` will be used as the default target.
  +targets = [ "x86_64-apple-darwin", "x86_64-pc-windows-msvc" ]
  # Additional `RUSTFLAGS` to set (default: none)
  rustc-args = [ "--example-rustc-arg" ]
  