I'm following a "keep it short" rule here.
In short, it's this:
- Instead of "this function returns blub" or "returns blub", write "blub". The function "is" blub in a sense.
- Replace "is true iff blub" by "blub".

From that perspective, phrases like "returns" and "is true if" seem a bit boilerplatey and redundant to me.
But if the general comment style in SymPy is different, I'll change it of course.