fix: enable proxy_cache_convert_head off and add tips at nginx config (#81)

Add important note about 'proxy_cache_convert_head off' in Nginx configuration to prevent S3 V4 signature verification failures.
Related Issue: rustfs/rustfs#717

Author: laozhoubuluo

docs/integration/nginx.md

@@ -54,7 +54,7 @@ server {
       proxy_set_header X-Forwarded-Proto $scheme;
 
       # Disable Nginx from converting HEAD to GET
-      # proxy_cache_convert_head off;
+      proxy_cache_convert_head off;
 
       proxy_connect_timeout 300;
       # Default is HTTP/1, keepalive is only enabled in HTTP/1.1
@@ -94,7 +94,7 @@ server {
       proxy_set_header X-Forwarded-Proto $scheme;
 
       # Disable Nginx from converting HEAD to GET
-      # proxy_cache_convert_head off;
+      proxy_cache_convert_head off;
 
       proxy_connect_timeout 300;
       # Default is HTTP/1, keepalive is only enabled in HTTP/1.1
@@ -115,6 +115,19 @@ server {
 
 ```
 
+#### Important Notice
+
+> [!WARNING]
+> **Key configuration items**
+> 
+> The `proxy_cache_convert_head off` directive **must be added** to the Nginx configuration for the following reasons:
+> 
+> - By default, Nginx converts HEAD requests to GET requests for caching.
+> - This conversion will cause S3 V4 signature verification to fail.
+> - The symptom is `Bucket not found` or `403 Access Denied` error when accessing the storage bucket.
+> 
+> Refer to [Nginx Official Documentation](http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_convert_head)。
+
 ## 3. Multi-Machine Load Balancing
 
 If you need to add multiple RustFS servers in a distributed environment, please adjust DNS resolution or local Hosts addresses in advance, and modify and add servers.
@@ -203,4 +216,4 @@ server {
       proxy_pass http://127.0.0.1:9001; 
    }
 }
-~~~
+~~~