doc: issue2551415 correct doc for change input->input_payload in 2.5 the rest interface changed a variable name from input to input_payload. An earlier commit changed the rest docs. This commit adds an item for it to the upgrading 2.4.0->2.5.0 section. Also cross reference added to the rest docs with the updated examples.

def init_severity(db, cl, nodeid, newvalues):
    """Make sure severity is set on new issues"""
    if 'severity' in newvalues and newvalues['severity']:
        return

    normal = db.severity.lookup('normal')
    newvalues['severity'] = normal

def init(db):
    db.issue.audit('create', init_severity)