Improve rule suppression documentation

[kapilmb]

Existing documentation doesn't provide any information regarding rule-wise behavior of code suppression.

For example, PSSA cannot suppress PSAvoidUsingUserNameAndPassWordParams violations based on given rulesuppressionid.

    Configuration Foo
    {             
     [Diagnostics.CodeAnalysis.SuppressMessageAttribute("PSAvoidUsingUserNameAndPassWordParams", "UserName", Scope="Function")]
     [Diagnostics.CodeAnalysis.SuppressMessageAttribute("PSAvoidUsingUserNameAndPassWordPara", "EncryptedPassword", Scope="Function")]
        param (
            [Parameter(Mandatory)]
            [String]$UserName,

            [Parameter(Mandatory)]
            [String]$Password
        )

    }