read-preference.txt lists commands that obey read preferences. 'text' should be added, at least when it comes to mongos.

Based on testing 2.4.4, it seems mongos obeys read preferences for the 'text' command. But I can't find the commit that implements this, so I'd like someone to verify that this is really true and for how long it has been so. The fix should go in read-preference.txt around line 500.

Note DRIVERS-92, not all drivers obey read preferences for 'text'.