docs: add HC4 to HC5 migration notes for resource naming

Document the default resource name format difference between HC4 and HC5
adapters so users understand that existing flow control rules will not
match automatically after migration, and provide a snippet for preserving
the legacy URI-only naming via a custom extractor.

Change-Id: Iadacf3d3288acb4b23ce67441a88c0b8ad0f0b6e
Co-developed-by: Claude <noreply@anthropic.com>

Author: uuuyuqi

sentinel-adapter/sentinel-apache-httpclient5-adapter/README.md

@@ -62,6 +62,32 @@ public class MyResourceExtractor implements ApacheHttpClientResourceExtractor {
 }
 ```
 
+### Migration from Apache HttpClient 4.x Adapter
+
+> **Note:** The default resource naming convention differs between the HC4 and HC5 adapters. If you are migrating from
+> `sentinel-apache-httpclient-adapter` to `sentinel-apache-httpclient5-adapter`, please review your existing flow control
+> rules carefully — they will **not** match automatically.
+
+| Adapter | Default Resource Name Format | Example |
+| --- | --- | --- |
+| `sentinel-apache-httpclient-adapter` (HC4) | URI only (path, may include query) | `httpclient:/api/users` |
+| `sentinel-apache-httpclient5-adapter` (HC5) | `METHOD:full_url` (query/fragment stripped) | `httpclient:GET:http://example.com/api/users` |
+
+Both adapters share the same default resource prefix (`httpclient:`), but the resource name suffix is incompatible.
+This means any flow control rules configured against HC4 resource names will no longer take effect after migration.
+
+You have two options:
+
+1. **Update your rules** to match the new HC5 resource name format (recommended — aligns with the OkHttp adapter and
+   provides better granularity by including the HTTP method).
+2. **Preserve the old format** by supplying a custom `ApacheHttpClientResourceExtractor` that returns only the URI path,
+   e.g.:
+
+   ```java
+   SentinelApacheHttpClientConfig config = new SentinelApacheHttpClientConfig();
+   config.setExtractor(request -> request.getRequestUri());
+   ```
+
 ### Fallback
 
 The default fallback throws `SentinelRpcException`. You can customize the behavior: