doc: document cwd field in flux-jobs(1)

Problem: The "cwd" field is not documented in flux-jobs(1).

Add documentation on it.

Author: chu11

doc/man1/flux-jobs.rst

@@ -302,6 +302,9 @@ The field names that can be specified are:
 **name**
    job name
 
+**cwd**
+   job current working directory
+
 **queue**
    job queue