validate_size_of implementation is confused

[BrucePerens]

The comments here confuse whether validate_size_of is supposed to validate the numeric value of something, or the size of something. This produces incorrect documentation at https://luckyframework.github.io/avram/Avram/Validations.html#validate_size_of(attribute:Avram::Attribute,min=nil,max=nil,allow_nil:Bool=false)-instance-method
This is also reflected in the incorrect documentation in the guides on the web site, see luckyframework/website#489

  # Validate the size of the attribute is within a `min` and/or `max`
  #
  # ```
  # validate_size_of age, min: 18, max: 100
  # validate_size_of account_balance, min: 500
  # ```
  def validate_size_of(
    attribute : Avram::Attribute,
    min = nil,
    max = nil,
    allow_nil : Bool = false
  )
    if !min.nil? && !max.nil? && min > max
      raise ImpossibleValidation.new(
        attribute: attribute.name,
        message: "size greater than #{min} but less than #{max}")
    end

    unless allow_nil && attribute.value.nil?
      size = attribute.value.to_s.size

[jwoertink]

Yeah, the docs on this are wrong. This method checks that a string length (size) is within a specific range.

Also related: #537 for adding a new method that does what these docs intended.