DocsHow to disable user registration (#7265)

### What problem does this PR solve?



### Type of change


- [x] Documentation Update

docker/.env

 # Note that neither `MAX_CONTENT_LENGTH` nor `client_max_body_size` sets the maximum size for files uploaded to an agent.
 # See https://ragflow.io/docs/dev/begin_component for details.
 
-# The log level for the RAGFlow's owned packages and imported packages.
-# Available level:
+# Log level for the RAGFlow's own and imported packages.
+# Available levels:
 # - `DEBUG`
 # - `INFO` (default)
 # - `WARNING`
 # - `ERROR`
-# For example, following line changes the log level of `ragflow.es_conn` to `DEBUG`:
+# For example, the following line changes the log level of `ragflow.es_conn` to `DEBUG`:
 # LOG_LEVELS=ragflow.es_conn=DEBUG
 
 # aliyun OSS configuration
 # REGION=cn-hangzhou
 # BUCKET=ragflow65536
 
-# user registration switch
+# A user registration switch:
+# - Enable registration: 1
+# - Disable registration: 0
 REGISTER_ENABLED=1

docs/configurations.md

 - `MACOS`  
   Optimizations for macOS. It is disabled by default. You can uncomment this line if your OS is macOS.
 
+### User registration
+
+- `REGISTER_ENABLED`
+  - `1`: (Default) Enable user registration.
+  - `0`: Disable user registration.
+
 ## Service configuration
 
 [service_conf.yaml.template](https://github.com/infiniflow/ragflow/blob/main/docker/service_conf.yaml.template) specifies the system-level configuration for RAGFlow and is used by its API server and task executor.

docs/release_notes.md

 - Agent version control: all updates are continuously logged and can be rolled back to a previous version via export.
 - Team collaboration: Agents can be shared with team members.
 - OpenAI-compatible APIs: Agents can be called via OpenAI-compatible APIs.
-- User registration control: administrators can enable or disable user registration through an environment variable.
+- User registration control: administrators can enable or disable user registration through an environment variable. See `REGISTER_ENABLED` in **/docker/.env**.
 
 ### Improvements
 
 
 - [Set page rank](./guides/dataset/set_page_rank.md)
 - [Enable RAPTOR](./guides/dataset/enable_raptor.md)
+- [Set variables for your chat assistant](./guides/chat/set_chat_variables.md)
 - [RAGFlow MCP server overview](./develop/mcp.md)
 
 ## v0.17.2