more doc cleanups

ucloud/docs/source/user-guide/how-to-create-an-os-image-for-ucloud.rst

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+How to create VM images for ucloud
+==================================
+
+Overview
+---------
+
+ucloud tries to be least invasise towards VMs and only require
+strictly necessary changes for running in a virtualised
+environment. This includes configurations for:
+
+* Configuring the network
+* Managing access via ssh keys
+* Resizing the attached disk(s)
+
+
+Network configuration
+---------------------
+All VMs in ucloud are required to support IPv6. The primary network
+configuration is always done using SLAAC. A VM thus needs only to be
+configured to
+
+* accept router advertisements on all network interfaces
+* use the router advertisements to configure the network interfaces
+* accept the DNS entries from the router advertisements
+
+
+Configuring SSH keys
+--------------------
+
+To be able to access the VM, ucloud support provisioning SSH keys.
+
+To accept ssh keys in your VM, request the URL
+*http://metadata/ssh_keys*. Add the content to the appropriate user's
+**authorized_keys** file. Below you find sample code to accomplish
+this task:
+
+.. code-block:: sh
+
+    tmp=$(mktemp)
+    curl -s http://metadata/ssk_keys > "$tmp"
+    touch ~/.ssh/authorized_keys # ensure it exists
+    cat ~/.ssh/authorized_keys >> "$tmp"
+    sort "$tmp" | uniq > ~/.ssh/authorized_keys
+
+
+Disk resize
+-----------
+In virtualised environments, the disk sizes might grow. The operating
+system should detect disks that are bigger than the existing partition
+table and resize accordingly. This task is os specific.
+
+ucloud does not support shrinking disks due to the complexity and
+intra OS dependencies.