PY-16906 Resolve references to global variables in module-level docstrings

PY-16906 Resolve references to global variables in module-level docstrings

Because DocStringParameterReference doesn't implement
PsiPolyVariantReference I had to adjust helper methods like
PyResolveTestCase#findReferenceByMarker and
PyMultiFileResolveTestCase#doResolve in resolve
tests to handle normal PsiReference reference under cursor.