I am looking at the documentation of the text files: https://docs.transifex.com/formats/txt
When I look at the file in translation mode on Transifex, I can see that there is a field for comments.
My questions are (numbered, so you can address them):
Whatever you add in your TXT source file will be considered translatable text and as a result, it will be available to your translators in Transifex Web Editor for translation. So, if you want to provide instructions to your translators about how they should translate your content, then I would suggest you use our instructions field in the editor. Instructions can either be added via Transifex Web Interface (check here for more details) or API.