Fix for missing merged auth docs, and add a quicker default selector for merging All auths

[Badgerati]

Description of the Change

Adds documentation which was missing, describing the need for -ScriptBlock on Merge-PodeAuth when -Valid All was passed.

Also adds a quick new -MergeDefault when -Valid All is passed, which acts as a faster shorthand for -ScriptBlock when we only want to use the User object returned by just one of the authentication methods.

Also fixes some of the merged auth examples, still using an older format.

Related Issue

Resolves #1179

Examples

Merge-PodeAuth -Name 'MergedAuth' -Authentication 'ApiKey', 'Basic' -Valid All -ScriptBlock {
    param($results)

    $apiUser = $results['ApiKey'].User
    $basicUser = $results['Basic'].User

    return @{
        User = @{
            Username = $basicUser.Username
            ID       = $apiUser.ID
            Name     = $apiUser.Name
            Type     = $apiUser.Type
            Roles    = @($apiUser.Roles + $basicUser.Roles) | Sort-Object -Unique
            Groups   = @($apiUser.Groups + $basicUser.Groups) | Sort-Object -Unique
        }
    }
}