Document that `load_only()` implies `undefer()`

I just noticed that my column property which is defined with deferred=True is automatically undeferred if I list the column in the load_only() option. While this makes sense (I wouldn't specify it if I didn't want it to be loaded) I just noticed it by coincidence - the name "load only" sounds like it just restricts what's loaded but doesn't enable additional loading.

An extra sentence in the load_only docs could clarify this...