Clarify if params should use capital letter in API doc

In !25201 (merged) it was suggested to use Boolean instead of boolean in REST API doc. I'm fine with either small or capitalized parameters in doc, but it would be great to be consistent, because:

• on most places in doc/api/ we use small letters (except auto-generated doc/api/graphql)
• https://docs.gitlab.com/ee/development/documentation/styleguide.html#method-description uses small letter in example too
• I can see benefit of being consistent and do not mix both styles (at least in the context of single document), mixing both together doesn't look great

The following discussion from !25201 (merged) should be addressed:

• @msedlakjakubowski started a discussion: (+2 comments)

  "Boolean" should be capitalized.

  | `confidential`                            | Boolean        | no       | Set an issue to be confidential. Default is `false`.  |

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Resolution: use "boolean" for consistency