[session] don't silently ignore commits commits while in the precommit or postcommit phase are silently ignored. This is OK in precommit since there'll be a commit afterwards anyway; OTOH in postcommit it may surprise the developer who didn't read the documentation for postcommit operations carefully enough. Related to #4383922.

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc