Clarify whether `now` excludes leap seconds

[bakkot]

The documentation of now says

    /// The returned timestamps represent the number of seconds since
    /// 1970-01-01T00:00:00Z, also known as [POSIX's Seconds Since the Epoch],
    /// also known as [Unix Time].

But those things are in contradiction: Unix time is currently 37 seconds behind the number of seconds since 1970-01-01T00:00:00Z.

This should be clarified, probably by adding "excluding leap seconds" to the description.