docs: rust: add paragraph about finding a suitable libclang

Sometimes users need to tweak the finding process of `libclang`
for `bindgen` via the `clang-sys`-provided environment variables.

Thus add a paragraph to the setting up guide, including a reference
to `clang-sys`'s relevant documentation.

Link: https://lore.kernel.org/rust-for-linux/CAKwvOdm5JT4wbdQQYuW+RT07rCi6whGBM2iUAyg8A1CmLXG6Nw@mail.gmail.com/
Reviewed-by: Nick Desaulniers <[email protected]>
Signed-off-by: Miguel Ojeda <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ojeda

Documentation/rust/quick-start.rst

@@ -100,6 +100,23 @@ Install it via (note that this will download and build the tool from source)::
 
 	cargo install --locked --version $(scripts/min-tool-version.sh bindgen) bindgen
 
+``bindgen`` needs to find a suitable ``libclang`` in order to work. If it is
+not found (or a different ``libclang`` than the one found should be used),
+the process can be tweaked using the environment variables understood by
+``clang-sys`` (the Rust bindings crate that ``bindgen`` uses to access
+``libclang``):
+
+* ``LLVM_CONFIG_PATH`` can be pointed to an ``llvm-config`` executable.
+
+* Or ``LIBCLANG_PATH`` can be pointed to a ``libclang`` shared library
+  or to the directory containing it.
+
+* Or ``CLANG_PATH`` can be pointed to a ``clang`` executable.
+
+For details, please see ``clang-sys``'s documentation at:
+
+	https://github.com/KyleMayes/clang-sys#environment-variables
+
 
 Requirements: Developing
 ------------------------