The attributes of fields of list views do not mention the `optional`
attribute while it is used by chapter 12 of the R&D Training.
This commit describes this `optional` attribute.
task-3380953
closesodoo/documentation#4908
X-original-commit: 5a446f5c59
Signed-off-by: Benoit Socias (bso) <bso@odoo.com>