docs: explain how to use global object identifier with functions

[FelixZY]

Description

@benjie additionally notes that the requirement to use the table record type (rather than the pk type) in the function definition will hopefully be lifted in the future. However, as this is not yet possible, I excluded this bit of "trivia".

resolves #2086

Performance impact

N/A

Security impact

N/A

Checklist

• [x] My code matches the project's code style and yarn lint:fix passes.
• [ ] I've added tests for the new feature, and yarn test passes.
• [x] I have detailed the new feature in the relevant documentation.
• [ ] I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• [ ] If this is a breaking change I've explained why.

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Latest commit: 60792decd305c49364fdc1aecf90f870e7ef75b0

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