Update doc/user/project/repository/gpg_signed_commits/index.md with clarifying gpg command for generating keys on other operating systems

See https://gitlab.com/gitlab-org/gitlab-ce/issues/36923.

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6492.
Signed-off-by: Rémy Coutable <remy@rymai.me>