Documentation update

[rjschwei]

Documentation update

• Pull cloud specific caveats into their own section
• Explicitly set the supported migration path for cloud instances
• Minor woring improvements
• Remove section about setting the migration target to make it a hidden feature

[schaefi]

An update process of the docs has been started here:

• https://bugzilla.suse.com/show_bug.cgi?id=1168446