eolian: inherit since information from struct and enum to field
If a struct or enum field doesn't explicitly sets since information, then since
is inherited from struct documentation if it is available.
Reviewers: jptiz, Jaehyun_Cho, woohyun, q66
Reviewed By: q66
Maniphest Tasks: T8359
Differential Revision: https://phab.enlightenment.org/D10948