Bug 11483: Add missing serial help files

The help files for the Serial Numbering Patterns and Frequencies pages
were missing.  This patch adds them.

To test:

* Visit the two new serial pages
* Click Help
* Confirm text and links are right

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 11483: (follow-up) fix typos

Signed-off-by: Galen Charlton <gmc@esilibrary.com>