Flip order of conditionals and add explanatory note

The conditional is nominally more performant and considerably more readable
with its constituents flipped. Also adds an explanatory note. Prompted by
confusion over change Ie89507f8e98c365df9d93a7633223103b9025790.

Change-Id: I2d3db63bdbcebe0fd59c063c9f371362c9d07f91