Add section to docs describing issue where old pymssql versions return no rows when used with new FreeTDS versions

This one has come up several times at SurveyMonkey and people sometimes get confused even about which versions have the problems (people often think the new pymssql can't work with an old FreeTDS, which is not true; it's only the old version of pymssql that can't work with the new FreeTDS :-))

Basically, I'm describing this issue: https://github.com/pymssql/pymssql/issues/137