Explicit notification of lack of support for rust-docs component for

Tier 2 targets

src/doc/rustc/src/platform-support.md

@@ -73,6 +73,8 @@ Tier Policy.
 
 All tier 2 targets with host tools support the full standard library.
 
+**NOTE:** Tier 2 targets currently do not build the `rust-docs` component.
+
 target | notes
 -------|-------
 `aarch64-apple-darwin` | ARM64 macOS (11.0+, Big Sur+)
@@ -112,6 +114,8 @@ The `std` column in the table below has the following meanings:
 
 [`no_std`]: https://rust-embedded.github.io/book/intro/no-std.html
 
+**NOTE:** Tier 2 targets currently do not build the `rust-docs` component.
+
 target | std | notes
 -------|:---:|-------
 `aarch64-apple-ios` | ✓ | ARM64 iOS