add Microsoft Exchange reply by email details
What does this MR do?
Document Reply by Email with Microsoft Exchange settings, fix https://gitlab.com/gitlab-org/gitlab-ce/issues/28131. Also includes minor cleanup and additions to Reply by Email docs:
- Document that not all features work without sub-addressing
- Remove redundant mailroom restart since omnibus already does that
Replace https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9356