doc: add id.emoji to flux-jobs(1)

Problem: `id.emoji` is missing from the list of available field names
in flux-jobs(1).

Add id.emoji to the list of valid field names in the flux-jobs(1)
manual.

Author: grondo

doc/man1/flux-jobs.rst

@@ -274,6 +274,9 @@ The field names that can be specified are:
 **id.words**
   job ID in mnemonic encoding
 
+**id.emoji**
+  job ID in emoji encoding
+
 **userid**
    job submitter's userid