Standardise documentation for pagination parameters

[hadley]

I've tried to document all parameters related to pagination in one place: the documentation for TWIT_paginate_max_id/TWIT_paginate_cursor. This means that the docs lose less specific details about the endpoint (e.g. specific page sizes and rate limits) but gain more general details about how all the pieces fit together.

@llrs I think the best place to focus on is the TWIT_paginate_max_id documentation, since that now flows out to the majority of functions in rtweet. Let me know if there's anything you think is confusing so that I can improve the wording as much as possible.

[llrs]

Should it be a numeric ID or a character? Not sure I understand well what this is but each message or status has an ID this ID increases as more content is published so if one wants content later than X message can use this?

[hadley]

Yes, that's right. But once #510 is done, the user shouldn't need to worry about the details. e.g. the code for retrieving more tweets from a search would be something like this:

tw <- search_tweets("#rstats", n = 10000)
tw_more <- search_tweets("#rstats", max_id = max_id(tw))

And that makes me realise why you might need since_id() — this code would get all the tweets since your last request:

tw_more <- search_tweets("#rstats", max_id = since_id(tw))

I'll make all this work in upcoming PRs.