doc: rust: explain rust-src needs to be re-added on upgrade

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

Author: ojeda

Documentation/rust/quick-start.rst

@@ -43,11 +43,17 @@ If ``rustup`` is being used, run::
 
 	rustup component add rust-src
 
+The components are installed per toolchain, thus upgrading the Rust compiler
+version later on requires re-adding the component.
+
 Otherwise, if a standalone installer is used, the Rust repository may be cloned
 into the installation folder of the toolchain::
 
 	git clone --recurse-submodules https://github.com/rust-lang/rust $(rustc --print sysroot)/lib/rustlib/src/rust
 
+In this case, upgrading the Rust compiler version later on requires manually
+updating this clone.
+
 
 libclang
 ********