doc: clarify adding ctx argument to check command in schema.py On IRC user was unsure what file was used for the 1.51 - 1.60 upgrade section: Enhancement to check command for Permissions added file name desription and example. Also marked it as optional. Clarified that if or when it becomes required there will be a new required upgrade direction.

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)