doc: process: be explicit about what the Quick Start guide explains

This tries to make the guide (and the target) as visible as possible.
It was taken from similar text in the Kconfig help option.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Documentation/process/changes.rst

@@ -93,7 +93,9 @@ is optional) needs to be installed to build the kernel. Other components are
 useful for developing.
 
 Please see :ref:`Documentation/rust/quick-start.rst <rust_quick_start>` for
-more information.
+instructions on how to satify the build requirements of Rust support. In
+particular, the Makefile target 'rustavailable' is useful to check why the Rust
+toolchain may not be detected.
 
 bindgen (optional)
 ------------------