Fixes #7586: Adds documentation for hints

[drslump]

Fixes #7586

Not sure about what all the hints mean, I've added them nonetheless so users can experiment on their own, it at least saves them from greping the compiler sources. But anyhow, if there are some that should be considered as private to the compiler I can remove them. And of course, please feel free to provide the missing explanations :)

[dom96]

Nice! Thank you for taking the time to compose this :)

[timotheecour]

• @drslump Would be great if documentation could be autogenerated from documentation comments of hints, to keep things DRY
• these documentation comments could also appear in nim --hints:list

[timotheecour]

@drslump while your PR improves the status quo, a better fix would be to auto-generate from each hint's documentation comments ; would be more DRY and doesn't get out of date (since it's compiler code it should be doable currently; but more generally for user code could be done via #8516 )