[book] typos / clarifications in create-instance section
Clarify that cubicweb-ctl upgrade needs to be run whenever cubicweb or a
cube is upgraded. Related to #2632425.
include *.pyinclude */*.pyrecursive-include data *.gif *.png *.ico *.css *.jsrecursive-include i18n *.porecursive-include wdoc *