man: extend on the usec/sec discrepancy

Let's document the discrepancy between the Sec and USec suffixing of
unit files and D-Bus properties at three places: in "systemctl show"
(where it already was briefly mentioned), in the D-Bus interface
description (at one place at least, i.e. the most prominent of
properties that encapsulate time values, there are many more) and in the
general man page explaining time values.

By documenting this at all three places I think we now do as much as we
can do about this highlighting the discrepancy of the naming and the
reasons behind it.

Fixes: #2047
(cherry picked from commit 3c719357)