cubicweb #2842345 doc: talk about the session.data API, not the deprecated get/set/del_session_data [validation pending]

see http://docs.cubicweb.org/devweb/request.html?highlight=session%20data

prioritynormal
typebug
done in3.17.4
load0.100
load left0.000
closed by#95e69c2d52a9 [doc] one must now manipulate the req.session.data dict (closes #2842345)
patch[doc] one must nos manipulate the req.session.data dict (closes #2842345) [applied]