The default value of thetzinfo argument todatetime.datetime is None, resulting in a "naive"datetime instance: the comment should state clearly that we forcibly convert such instances toUTC. E.g.:

    :paramexpiration:PointintimewhenthesignedURLshouldexpire.Ifa`datetime`instanceispassedwithoutanexplicit`tzinfo`set,itwillbeconvertedto``UTC``.

Rather than "it will be converted to" let's use "it will be assumed to be". This is a huge footgun; a user who sees the term "converted" will assume that we will do the right thing in the default case, whereas in fact we will almost always do the wrong thing.