docs: some small wording changes to the Gateway document

[vladfrangu]

I'd love some input for the Identifying section - the maximum concurrency is mentioned 3 times in the document: once in Identifying, once in Rate Limits and once in its own section Max concurrency. What would be the ideal move to hopefully remove some of this duplication (or at least move it closer so you don't skip half the page to read about it)? 😅