Document why REST editmeta requires Issue Key/ID

Summary

All issues of the same issue type in the same project use the same schemes. This means, for editmeta REST endpoint, it's only necessary to supply project and issuetype to retrieve all the editable fields for all issues in that project and of that issue type. Permissions shouldn't matter here and should only be checked when an actual issue update is performed.

Suggestion

Currently, editmeta requires an issue key/ID, which is inconvenient and slows down the process. It should work in the same way as createmeta.

If an issue key/ID must be included, please clearly document why.