add help text for Certificate Manager serial number
I got confused by the 'Serial' field label in the CA manager for a CA certificate. I figured it was the CA's serial, but that was a bad assumption:
Attaching a suggested patch with expanded help text to help the next guy.
n.b. I just mimicked the existing instances, but based on indentation levels, at least, I think there's an extraneous </td> at the end of each bit of help text after the <br>[help text] entries in this file. I didn't run this down extensively.