Update docs and diagnostic message for UseCorrectCasing (#2138)

* Inline the 2 overloads of GetDiagnosticRecord to their individual call sites

It made it cumbersome to pass a different number of arguments to the string.Format call and didn't seem necessary

* Add tests for diagnostic messages related to casing of functions, parameters, and operators

* Remove errant space in rule name

* Swap over and expand descriptions of CheckCommands and CheckOperator

* Update the 'Wrong way' section to use an operator in the wrong way

* inline local message var

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Co-authored-by: Christoph Bergmeister <c.bergmeister@gmail.com>