[Docs] Clarify write-to-file and read-from-file Behavior

[robertDurst]

Addresses: racket/racket#5224

If a user discovers write-to-file or read-from-file and plans to leverage either without serendipitously also discovering the accompanying editor header/footer methods, they'll likely have a challenging time.

Beyond simply clarifying behavior and linking to header/footer related documentation, also included @alex-hhh 's complete example within the docs for others to discover.

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My first contribution. As this is documentation, no accompanying unit-tests or changelog/history updates are required.

Verification

• raco setup ran successfully
• docs looked good locally

[racket-discourse-github-bot]

This pull request has been mentioned on Racket Discourse. There might be relevant details there:

https://racket.discourse.group/t/how-to-serialize-deserialize-text-editors-and-structures-including-text-editors/3605/8

[mflatt]

Thanks! I updated the branch to avoid a merge conflict and to shift the "See also File Format" to after your explanation.