Clarify casing of I18N placeholder names
What does this MR do?
Document how placeholder names in translatable strings should be picked.
What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15846#note_50923388
Document how placeholder names in translatable strings should be picked.
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15846#note_50923388