doc: document {id.dec} in flux-jobs(1)

Problem: The output format of {id.dec} was not documented
in flux-jobs(1).

Add it to the list of id output formats.

Author: chu11

doc/man1/flux-jobs.rst

@@ -202,6 +202,9 @@ The field names that can be specified are:
 **id.f58**
   job ID in RFC 19 F58 (base58) encoding
 
+**id.dec**
+  job ID in decimal representation
+
 **id.hex**
    job ID in ``0x`` prefix hexadecimal representation