rust: Document ppc64le RUST_OPT_LEVEL_0/1 restrictions

Mention that RUST_OPT_LEVEL_0/1 require increasing the kernel stack
size.

Signed-off-by: Michael Ellerman <[email protected]>

Author: mpe

Documentation/rust/arch-support.rst

@@ -23,7 +23,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
      - None
    * - ``powerpc``
      - Maintained
-     - ``ppc64le`` only, ``RUST_OPT_LEVEL >= 2``
+     - ``ppc64le`` only, ``RUST_OPT_LEVEL < 2`` requires ``CONFIG_THREAD_SHIFT=15``
    * - ``x86``
      - Maintained
      - ``x86_64`` only