jit: further doc fixes

Further jit doc fixes, which fix links to
gcc_jit_function_type_get_param_type and gcc_jit_struct_get_field.

gcc/jit/ChangeLog:
	* docs/topics/expressions.rst: Fix formatting.
	* docs/topics/types.rst: Likewise.
	* docs/_build/texinfo/libgccjit.texi: Regenerate

Signed-off-by: David Malcolm <dmalcolm@redhat.com>