Document "Maximum artifacts size" setting in Admin

The "Maximum artifacts size" setting is undocumented. This was introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1584

This seems to be creating some confusion, see https://gitlab.com/gitlab-org/gitlab-ce/issues/14841#note_12001981

When searching for the error that occurs as a result of the maximum size limit

ERROR: Uploading artifacts to coordinator... too large archive id=94 responseStatus=413 Request Entity Too Large status=413 Request Entity Too Large token=

The most common results point to. The setting seems to be pointed out in each issue.

• https://gitlab.com/gitlab-org/gitlab-ce/issues/3812
• https://gitlab.com/gitlab-org/gitlab-ce/issues/3607#note_2740610
• https://gitlab.com/gitlab-org/gitlab-ce/issues/3643#note_2756825

The setting should be documented to avoid future confusion.

//cc @axil @virtuacreative