Add recommendation for FORM.MEDI usage

The proposed recommendation is consistent with sample files:
* https://gedcom.io/testfiles/gedcom70/maximal70.ged has MEDIA, OTHER,
  and ELECTRONIC.
* FamilySearch/GEDCOM.io#151 adds obje-1.ged
  with PHOTO and VIDEO, as copied from
  https://github.com/gedcom7code/test-files/blob/main/7/obje-1.ged

Fixes FamilySearch#316 since it addresses the last piece that wasn't already done

Signed-off-by: Dave Thaler <dthaler@armidalesoftware.com>

Author: dthaler2

specification/gedcom-3-structures-3-meaning.md

@@ -872,6 +872,13 @@ Consider an asset in a repository that is a digital scan of a book of compiled n
 for this asset, the `CALN`.`MEDI` is recommended to be `ELECTRONIC` rather than `BOOK` or `NEWSPAPER`.
 :::
 
+When `MEDI` is a substructure of a `g7:FORM`, it is recommended that its payload describes the medium from which it was derived.
+
+:::example
+Consider a digital photo in a multimedia record;
+for this asset, the `FORM`.`MEDI` is recommended to be `PHOTO` rather than `ELECTRONIC`.
+:::
+
 #### `MIME` (Media type) `g7:MIME`
 
 Indicates the [media type](#media-type) of the payload of the superstructure.