Cppcoreguidelines: Improve Non-Rule readability

[dksmiffs]

Format suggestion: "NR.#: Don't: [Active verb] [rest of non-rule here]"

Suggest rewording the following non-rules' titles to make them more readable:

• NR.1: Don't: Place all declarations at the top of a function
• NR.3: Don't: Avoid exceptions
• NR.5: Don't: Avoid substantive work in a constructor; instead use two-phase initialization

[cubbimew]

I agree that double negation is confusing.

[hsutter]

Editors call: We agree these could be reworded better to avoid double negation.