Update EF Core docs for one-to-one relationships

[bkoelman]

Follow-up for #1118 (comment).

QUALITY CHECKLIST

• N/A: Changes implemented in code
• N/A: Complies with our contributing guidelines
• N/A: Adapted tests
• Documentation updated

[codecov]

Codecov Report

Merging #1207 (1ed1ac8) into master (af46168) will not change coverage.
The diff coverage is n/a.

❗ Current head 1ed1ac8 differs from pull request most recent head bbec95b. Consider uploading reports for the commit bbec95b to get more accurate results

@@           Coverage Diff           @@
##           master    #1207   +/-   ##
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  Coverage   92.69%   92.69%           
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  Files         243      243           
  Lines        7803     7803           
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  Hits         7233     7233           
  Misses        570      570           

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