Clarify explanation of "differentials"

[fedarko]

I think some of the output fields you get from some tools (e.g. ALDEx2) aren't exactly log-fold changes -- stuff like the p-values included in the FeatureData[Differential] output (e.g. we.ep) may still be useful for inspecting in a rank plot but should probably be explained as not really being "differentials" in the sense defined in the reference frames paper --

The term “differential” refers to the logarithm of the fold change in abundance of a taxa between two conditions.

It would be a good idea to clarify this in the Qurro docs / in the ALDEx2 tutorial so that users aren't confused. Ties in to #229.

@mortonjt tagging in case you have any opinions on this.