Expand note on :errors

[nathanl]

Also opened a PR to link here in the Ecto docs: https://github.com/elixir-ecto/ecto/pull/3980

[nathanl]

On August 16 in the Elixir Slack, @chrismccord said about this PR:

the doc changes may be a bit too specific because the form errors are shown conditionally in the template by reading the changeset directly in the cast of dead views

I said:

You mean here? https://github.com/phoenixframework/phoenix_ecto/blob/2cf35cf2e441416bfb82b72aafc1659a12873cd2/lib/phoenix_ecto/html.ex#L292-L294 I don't get why that makes the doc changes too specific - can you elaborate?

[josevalim]

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