Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/30473
MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14046

And link to it from where you change your username.

And expand on the help text.

Add Security Concerns section to reply by email documentation

[ci skip]

See merge request gitlab/gitlabhq!2191

* Improve the descriptive text
* Rename "from" to "Target" and "to" to "Source"
* Swap "Target" and "Source" to have the same order as in MRs
* Reworded "Switch base of comparison" to "Swap versions"

Signed-off-by: Rémy Coutable <remy@rymai.me>