The user image documentation applies to both standalone and glance
usage, but we keep it in the glance section. This change creates
a new page to talk about user images.
We also don't emphasize the difference between partition and whole
disk images enough, resulting in users trying whole disk images
with kernel/ramdisk set. Make the difference clearer.
Change-Id: I7185679203e0f9aa7ebe9c69971009cd59271b89
530ec9d0b1