Cleanup launch instance and manage IPs docs

This is a follow up to Ie039322660fd0e2e0403843448379b78114c425b.

A few things are changed here:

* The note about using file injection is removed. File injection
  was deprecated in the API in Queens and not something that we
  really want users using.
* Mention that creating a flavor is typically admin-only.
* Link to the BDM docs for more details about BDM parameter values.
* Update the manage-ip-address docs to make the examples rely on
  using the networking resource CLIs rather than any proxy APIs
  that were available in nova.

Change-Id: Ifa2e2bbb4c5f51f13d1a5832bd7dbf9f690fcad7

doc/source/user/launch-instance-from-volume.rst

@@ -250,6 +250,8 @@ the volume to boot an instance.
 
    - ``NAME``. The name for the server.
 
+   See :doc:`block-device-mapping` for more details on these parameters.
+
 #. Create a bootable volume from an image. Cinder makes a volume bootable
    when ``--image`` parameter is passed.