Update REST API documentation to differentiate between username vs user_key

The REST API documentation (e.g. https://docs.atlassian.com/software/jira/docs/api/REST/7.12.0/#api/2/project/) uses the terms user_key and username interchangeably. This leads to confusion because both values exist in the database schema and one value over the other fails.

If the endpoints can't be changed to accept username (instead of userkey) then I'd also suggest careful use of the relevant term in each endpoint's description and a description of the difference earlier in the documentation.

An explanatory sentence of this bug (JRASERVER-68189) on the REST documentation would be really useful or a more specific error message when the incorrect value is used.