在创建或更改DocumentDB密码时，确保密码长度不超过最大值。根据AWS官方文档，AWS DocumentDB密码长度不能超过64个字符。以下是一个示例代码，可以在创建数据库实例时设置密码并检查密码长度是否符合规定：

import boto3
from botocore.exceptions import ClientError

# Set initial variables
password = 'mydbpassword'

# Create a DocumentDB client
client = boto3.client('docdb')

# Create a DB instance
try:
    response = client.create_db_instance(
        DBInstanceIdentifier='mydbinstance',
        MasterUsername='mydbuser',
        MasterUserPassword=password,
        DBInstanceClass='db.r5.large',
        Engine='docdb',
        AvailabilityZone='us-west-2a'
    )
    print('Database instance created')
except ClientError as e:
    print(e)

# Check password length
if len(password) > 64:
    print('Password length is greater than maximum')
else:
    print('Password length is within limit')

在以上代码中，如果密码超过64个字符，将会打印出“Password length is greater than maximum”。如果密码长度符合要求，则会打印出“Password length is within limit”。