Update onie-bisdn-upgrade documentation

[ideaship]

Update man page for onie-bisdn-upgrade:

• drop description of IMAGEURL option which has been removed
• make title like look more like a "standard" man page
• tweak description in name section
• avoid justified text with long URL, it ends up looking something like "URL of an installable BISDN Linux image (example:"
• move usage example to new examples section
• replace sample URLs with ones that look more like our current repo URLs

Update onie-bisdn-upgrade script:

• replace sample URLs with ones that look more like our current repo URLs
• slight reformatting of examples

[ideaship]

I pushed an update that should address all review comments.