WRN_ParamsArrayInLambdaOnly doesn't have single quotes around its argument.

  <data name="WRN_ParamsArrayInLambdaOnly" xml:space="preserve">
    <value>Parameter {0} has params modifier in lambda but not in target delegate type.</value>
  </data>

This is inconsistent with other diagnostics that we produce. Also the arguments blends in in the message. For example:

                // (67,49): warning CS9100: Parameter 1 has params modifier in lambda but not in target delegate type.
                //         Action<string[]> q17 = (params string[] s)=>{};
                Diagnostic(ErrorCode.WRN_ParamsArrayInLambdaOnly, "s").WithArguments("1").WithLocation(67, 49),

Another aspect to consider is, if we always use parameter number for the argument, then it might make sense to include the word "number" into the message. Number "1" could easily be confused with letter "l", for example, and interpreted as a name instead.