Fix the documentation of opam lint --warnings

[kit-ty-kate]

The documentation for opam lint --warnings=WARN currently states that when WARN starts with a + the default warnings are disables, however this is also the case with @ (added in #5652)

cc @Khady maybe that would have helped with your problem where it wasn't clear to you that @ is a replacement to + and not an addition (not -43+@21 but -43@21)

Or maybe should should have a description more like ocamlc's as it is basically the same thing:

  -w <list>  Enable or disable warnings according to <list>:
        +<spec>   enable warnings in <spec>
        -<spec>   disable warnings in <spec>
        @<spec>   enable warnings in <spec> and treat them as errors
     <spec> can be:
        <num>             a single warning number
        <num1>..<num2>    a range of consecutive warning numbers
        <letter>          a predefined set
     default setting is "+a-4-7-9-27-29-30-32..42-44-45-48-50-60-66..70"

[Khady]

looks good to me, thanks. Now that I re-read the explanation carefully it's obvious that @ is an "operator" like + or -. I wasn't careful enough. The description from ocamlc is maybe easier to understand for someone who just gives a very quick look to the doc but isn’t as complete?