DOC: clarify (un)aware logic in tz_localize() docstring
A simple hopefully no-brainer documentation patch; came across this while reading https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DatetimeIndex.tz_localize.html#pandas-datetimeindex-tz-localize.

Pushed another commit, PTAL @mroeschke!  thanks @jgehrcke 