Improve merge request description placeholder

From gitlab-design#441 (closed).

The merge request form “Description” field says “Write a comment…” which doesn't make sense. One supposes this is where they should describe their work.

We should improve the MR description placeholder to encourage best practice of describing the changes and anything the reviewers should be aware of.

Proposal

Change “Description” field placeholder to Describe the goal of the changes and what reviewers should be aware of.