Mention "Require long-form partial rendering" in the v2 upgrade guide

[Burgestrand]

https://github.com/phlex-ruby/phlex-rails/issues/137 https://github.com/phlex-ruby/phlex-rails/pull/223 https://github.com/phlex-ruby/phlex-rails/issues/196

This changes behaviour when rendering in a layout, i.e. render "chat_init" now becomes the literal string "chat_init" instead of rendering the partial as it did in v1.