Clarify Markdown vs MarkdownV2 support

[shardwolfdotnet]

The docs mention MarkdownV2 support, but based on my own tests it appears that only legacy Markdown syntax is supported.

For example, see the following actions snippet:

jobs:
  send_message:
    runs-on: ubuntu-latest
    steps:
      - name: Telegram Message Notify
        uses: appleboy/telegram-action@v0.1.1
        with:
          to: ${{ secrets.TELEGRAM_TO }}
          token: ${{ secrets.TELEGRAM_BOT_TOKEN }}
          message: |
            ${{ github.actor }} ${{ github.event.action }} PR ${{ github.event.number }}:
            [_${{ github.event.pull_request.title }}_](${{ github.event.pull_request.html_url }})
          format: markdown

Despite not being escaped, the italicized title didn't work as expected:

The markdown style notes mention this:

Entities must not be nested, use parse mode MarkdownV2 instead.

Moreover, drone-telegram seems to indicate markdown (v1) is the supported Markdown flavor - not v2.

Can the docs get updated to indicate which flavor of markdown is actually supported?