jit: remove "Alpha" warning from docs

I removed the "Alpha" warning from the JIT wiki page on
2020-05-18:
  https://gcc.gnu.org/wiki/JIT?action=diff&rev1=47&rev2=48
but forgot to remove it from the documentation, which this
patch does.

gcc/jit/ChangeLog:
	* docs/cp/index.rst: Remove "Alpha" warning.
	* docs/index.rst: Likewise.
	* docs/_build/texinfo/libgccjit.texi: Regenerate