Add patch for text OR markdown text to chat.* methods

[brendo]

RE: slack-ruby/slack-ruby-client#579

This PR adjusts the patches for the chat methods to accept markdown_text, but only if it's provided exclusively of text.

I'm not thrilled with the wording so open to suggestions 🙇

Locally, when using this code to generate slack-ruby-client this produces this output (one example provided):

  def chat_postMessage(options = {})
    raise ArgumentError, 'Required arguments :channel missing' if options[:channel].nil?
    raise ArgumentError, 'At least one of :attachments, :blocks, :text, :markdown_text is required' if options[:attachments].nil? && options[:blocks].nil? && options[:text].nil? && options[:markdown_text].nil?
    raise ArgumentError, 'Exactly one of :text, :markdown_text is required' unless options[:text].nil? ^ options[:markdown_text].nil?
    options = encode_options_as_json(options, %i[attachments blocks metadata])
    post('chat.postMessage', options)
  end

[dblock]

Is "One of these arguments in addition to text or markdown_text ..." and "text or markdown_text will be used" better?

[brendo]

Is "One of these arguments in addition to text or markdown_text ..." and "text or markdown_text will be used" better?

Heh, I was actually more concerned about the desc text (eg. https://github.com/slack-ruby/slack-api-ref/pull/81/files#diff-bab7373208eb0c66cc06b17136d209fed196ecce690150e93ccbe718b03eb391R10). The error message text is auto-generated so I don't believe we have the flexibility to change it for this specific use-case?

[dblock]

@brendo That's what I meant too.

Replace

"desc": "One of these arguments in addition to text (unless markdown_text is provided) is required to describe the content of the message. When attachments or blocks are included, text will be used as fallback text for notifications only.",

with

"desc": "One of these arguments in addition to either text or markdown_text is required to describe the content of the message. When attachments or blocks are included, text or markdown_text will be used as fallback text for notifications only.",

[brendo]

Sorry about that! Done :)

[dblock]

Merged via b1be09b.