API doc scaffolding: allow to specifiy the type of an optional param
API doc scaffolding: allow to specifiy the type of an optional param

Rationale: there is no reason why this should be allowed for arguments, but not
for parameters. On the other hand, this change only allows to *pass* the type,
but doesn't make the API doc engine actually *render* the information.

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui