doc: in flux-jobs(1) add D conversion flag info

Problem: The D conversion flag will convert both empty string and
epoch time (0) to an empty string.  But the epoch time conversion
is not mentioned.

Add note that epoch time will also be converted to empty string
by the D conversion flag.

Author: chu11

doc/man1/flux-jobs.rst

@@ -240,7 +240,8 @@ the following conversion flags are supported by :program:`flux jobs`:
 
 **!D**
    convert a timestamp field to ISO8601 date and time (e.g. 2020-01-07T13:31:00).
-   Defaults to empty string if timestamp field does not exist.
+   Defaults to empty string if timestamp field does not exist or the timestamp
+   is 0 (i.e epoch time).
 
 **!d**
    convert a timestamp to a Python datetime object. This allows datetime