Add warning to arm-none-eabi.md instead of a Note:

Author: thejpster

src/doc/rustc/src/platform-support/arm-none-eabi.md

@@ -105,10 +105,14 @@ features you do not have available, leaving you with the optimized instruction
 scheduling and support for the features you do have. More details are available
 in the detailed target-specific documentation.
 
-**Note:** Many target-features are currently unstable and subject to change, and
+<div class="warning">
+
+Many target-features are currently unstable and subject to change, and
 if you use them you should disassemble the compiler output and manually inspect
 it to ensure only appropriate instructions for your CPU have been generated.
 
+</div>
+
 If you wish to use the *target-cpu* and *target-feature* options, you can add
 them to your `.cargo/config.toml` file alongside any other flags your project
 uses (likely linker related ones):