Clarify API documentation

What does this MR do?

Some of the API documentation is REST-specific, but is written to look generic. (That is probably because it was written when the REST API was the only API.) This changes some of that generic-looking text to clarify it is REST-specific. The important thing to note is that most of the content on the main API documentation page is REST-specific. That is a problem we should solve in a future MR. This MR is intended to add clarity prior to solving that problem.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.