📝 Add OpenAPI warning to "Body - Fields" docs with extra schema extensions

[ml-evs]

This PR adds a warning in the docs about generating invalid OpenAPI schemas (see #3745) when using additional keys in pydantic Field specifications. This warning only effects versions fastapi>=0.66, which could be added to the note if helpful.

I would have liked to include a workaround for those who want to keep generating valid schemas, but is currently non-trivial and either involves overriding a whole chain of field info classes, or writing a custom OpenAPI schema method, to undo the effect of extra = "allow" in the fastapi.openapi.models.Schema class.
It is a bit of a shame this hasn't been acknowledged as an issue in #3745 but I hope that this warning will be helpful for those who are trying to use generic OpenAPI tools with FastAPI.

[codecov]

Codecov Report

Merging #4846 (1d9b364) into master (146f57b) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #4846   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files          531       531           
  Lines        13627     13629    +2     
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+ Hits         13627     13629    +2     

Impacted Files	Coverage Δ
fastapi/exceptions.py	100.00% <0.00%> (ø)
fastapi/applications.py	100.00% <0.00%> (ø)
fastapi/dependencies/utils.py	100.00% <0.00%> (ø)

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[github-actions]

📝 Docs preview for commit c8182fb at: https://626e94c817e90a569d91b459--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 1d9b364 at: https://627c2d135aa774107ddd9516--fastapi.netlify.app

[tiangolo]

Thanks @ml-evs! I updated the tip/warning a bit to avoid mentioning const as that would mean it would also have to explain how const is valid JSON Schema v6 but not valid OpenAPI 3.0.x, although as OpenAPI 3.1.x is based on JSON Schema v7 then it is compatible there.