jit: document union types

jit: document union types

gcc/jit/ChangeLog:
	* docs/topics/types.rst (gcc_jit_context_new_union_type): Add
	documentation.
	* docs/_build/texinfo/libgccjit.texi: Regenerate.

gcc/testsuite/ChangeLog:
	* jit.dg/test-accessing-union.c: Add comments for use by
	gcc/jit/docs/topics/types.rst.

From-SVN: r225246