kernel-doc/rst: add support for &union foo and &typedef foo references

Let the user use "&union foo" and "&typedef foo" to reference foo. The
difference to using "union &foo", "typedef &foo", or just "&foo" (which
are valid too) is that "union" and "typedef" become part of the link
text.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>