Upstream task: https://secure.phabricator.com/T4316
Phabricator discourages explaining non-trivial changes to a task.
There is no way to leave a comment from the task editing interface. There is no way to make non-trivial changes to a bug report while leaving a comment. This encourages doing things like rephrasing the summary or reassigning projects without leaving a comment about it, or doing them in several steps (using the minimal interface above comment field), which can be annoying to people watching the bug. (It sure annoys me.)