docs: add a more precise example in enum doc (GH-121015)

* docs: add a more precise example

Previous example used manual integer value assignment in class based declaration but in functional syntax has been used auto value assignment what could be confusing for the new users. Additionally documentation doesn't show how to declare new enum via functional syntax with usage of the manual value assignment.

* docs: remove whitespace characters

* refactor: change example

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Co-authored-by: Ethan Furman <ethan@stoneleaf.us>