Update Rest API documentation to clarify about /rest/api/2/user/properties/{propertyKey}

Problem Definition

Currently JIRA 7 has a new REST API call for user properties. However, as also already stated by the dev in this comment, the documentation is not clear on the specifics of this call as explained by the dev in the comment above.

Therefore users would be attempting to access properties of users created via UI using REST API and also will expect to access properties created via REST API from the UI. However, this is an intended behavior as explained in the above ticket.

Suggested Solution

Please update the doc to clarify the rest call as explained by the dev