doc: document updates to !d conversion specifier

Problem: The documentation of `!d` handling in flux-jobs(1) is out
of date.

Update the documentation to include that `!d` formats to an empty string
when unset (is 0.), and that `h` is supported.

Author: grondo

doc/man1/flux-jobs.rst

@@ -221,8 +221,8 @@ the following conversion flags are supported by *flux-jobs*:
    convert a timestamp to a Python datetime object. This allows datetime
    specific format to be used, e.g. *{t_inactive!d:%H:%M:%S}*. Additionally,
    width and alignment can be specified after the time format by using
-   two colons (``::``), e.g. *{t_inactive!d:%H:%M:%S::>20}*. Defaults to
-   datetime of epoch if timestamp field does not exist.
+   two colons (``::``), e.g. *{t_inactive!d:%H:%M:%S::>20}*. Returns an
+   empty string (or "-" if the *h* suffix is used) for an unset timestamp.
 
 **!F**
    convert a time duration in floating point seconds to Flux Standard