man: update note on flux-jobs(1) output

Problem: The --no-header option in flux-jobs may also affect
truncation warnings, not just the header.

Add a note about this in the flux-jobs(1) manpage.

Author: chu11

doc/man1/flux-jobs.rst

@@ -36,7 +36,8 @@ OPTIONS
 
 .. option:: -n, --no-header
 
-   For default output, do not output column headers.
+   For default output, do not output column headers and truncation
+   warnings.
 
 .. option:: -u, --user=[USERNAME|UID]