standard repos: allow conversion from handle and improve information

standard repos: allow conversion from handle and improve information

Add a description for the handle, which can be useful to display
alongside the name. The descriptions are essentially the first
sentence from PVE's "Package Repositories" docs, but without the
product name.

Also drop the " Repository" suffix from the names, as it's not useful,
but can be ugly: e.g. for the UI when the label already is
'Repository:'.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>