fix(gatsby): improve deprecation text for missing childOf directive

[vladar]

Description

This PR addresses a confusing deprecation warning when the @childOf directive is not set for some parent. Warning text before this PR:

The type `Parent` does not explicitly define the field `childChild`.
On types with the `@dontInfer` directive, or with the `infer ` extension set to `false`, automatically adding fields for
children types is deprecated. In Gatsby v3, only children fields explicitly set with the `childOf` extension will be added.`

After this PR:

Deprecation warning: In Gatsby v3 field `Parent.childChild` will not be added automatically
because type `Child` does not explicitly list type `Parent` in `childOf` extension.
Add the following type definition to fix this:

  type Child implements Node @childOf(types: ["Parent"]) {
    id
  }

https://www.gatsbyjs.com/docs/actions/#createTypes

Related issues

See #19674 (comment) as one example where it was confusing

Also related PR: #28483 (it fixes this warning in gatsby-plugin-schema-snapshot):

[pieh]

Published in [email protected]