[DOCS][ESQL] Document case insensitive equals operator

[leemthompo]

Blocked because currently disabled. See #105603.

• Closes ESQL: docs for =~ operator #104844
• Operator added in ESQL: add =~ operator (case insensitive equality) #103656

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[leemthompo]

🚗

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[luigidellaquila]

Thank you very much @leemthompo, I left a comment about a small improvement.

Partially unrelated to this PR, I also noticed that the type tables for binary operators only report integer, integer as valid inputs, while most of the operators accept any type, and =~ does not accept numbers at all.

I think it's something we'll have to double-check, probably @nik9000 has a quick pointer

[leemthompo]

Partially unrelated to this PR, I also noticed that the type tables for binary operators only report integer, integer as valid inputs, while most of the operators accept any type, and =~ does not accept numbers at all.

@luigidellaquila I'll create an issue to look into this separately 👍

[leemthompo]

@nik9000 not sure how to proceed here with regards the railroad diagrams

[nik9000]

I think it's something we'll have to double-check, probably @nik9000 has a quick pointer

This is because we don't have tests for those types. @not-napoleon is working on that.

[nik9000]

The reason we don't have a railroad diagram and type table is that those come from building a subclass of AbstractFunctionTestCase for your function. @luigidellaquila, could you do that?