migration: Mention that job resource must exist.

The attempt to migrate a job whose job resource has been removed from
the director configuration fails with an error like

        Fatal error: Previous Job resource not found for "rur_system_job"

It is not obvious that the job resource is required to migrate a
job, so this commit adds a new item for this fact in the "Important
Migration Considerations" bullet list.

manuals/en/main/migration.tex

@@ -115,6 +115,10 @@ \section{Important Migration Considerations}
       This policy also prevents deadlock situations, like attempting to
       read and write the same Volume from two jobs at the same time.
 
+\item Migration works only if the Job resource of the original Job is
+      still defined in the current director configuration. Otherwise
+      you'll get a fatal error.
+
 \item As noted above, for the Migration High Bytes, the calculation
       of the bytes to migrate is somewhat approximate.