docs: rust: add "The Rust experiment" section

Clarify that the Rust experiment is still going on to avoid
confusion for both kernel maintainers and end users.

Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
Link: https://lore.kernel.org/r/20231018160922.1018962-1-ojeda@kernel.org
[ Changed last paragraph as discussed in the mailing list. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Documentation/rust/index.rst

@@ -6,6 +6,25 @@ Rust
 Documentation related to Rust within the kernel. To start using Rust
 in the kernel, please read the quick-start.rst guide.
 
+
+The Rust experiment
+-------------------
+
+The Rust support was merged in v6.1 into mainline in order to help in
+determining whether Rust as a language was suitable for the kernel, i.e. worth
+the tradeoffs.
+
+Currently, the Rust support is primarily intended for kernel developers and
+maintainers interested in the Rust support, so that they can start working on
+abstractions and drivers, as well as helping the development of infrastructure
+and tools.
+
+If you are an end user, please note that there are currently no in-tree
+drivers/modules suitable or intended for production use, and that the Rust
+support is still in development/experimental, especially for certain kernel
+configurations.
+
+
 .. only:: rustdoc and html
 
 	You can also browse `rustdoc documentation <rustdoc/kernel/index.html>`_.