Clarify API descriptions of ID formats

What does this MR do?

Updates API documentation on these pages, to make clearer how the ID field can be formatted:

• doc/api/merge_request_approvals.md
• doc/api/notification_settings.md
• doc/api/project_clusters.md
• doc/api/project_snippets.md
• doc/api/projects.md
• doc/api/resource_access_tokens.md

@katrinleinweber, @.luke, I want to be clear that this MR is a little bare-bones. I've updated the ID definitions that Katrin identified, but I haven't attempted to add in any other language like Luke mentioned #331237 (comment 596140456):

Maybe we want to add a section that discusses this to the main REST API doc (a bit similar to this clarification: https://docs.gitlab.com/ee/api/#id-vs-iid)

I'd need a rough draft from Luke before I can add that, but these changes might be enough on their own?

Related issues

Closes #331237 (closed)

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