message: Document `RUSTDOCFLAGS`
diff: @@ -22,6 +22,9 @@ system:
  invocation, with the first argument being rustc.
  * `RUSTDOC` - Instead of running `rustdoc`, Cargo will execute this specified
  `rustdoc` instance instead.
  +* `RUSTDOCFLAGS` - A space-separated list of custom flags to pass to all `rustdoc`
  + invocations that Cargo performs. In contrast with `cargo rustdoc`, this is
  + useful for passing a flag to *all* `rustdoc` instances.
  * `RUSTFLAGS` - A space-separated list of custom flags to pass to all compiler
  invocations that Cargo performs. In contrast with `cargo rustc`, this is
  useful for passing a flag to *all* compiler instances.
  