[guides] provided missing information on how to enable quantization o…

…n existing collections (#1426)

qdrant-landing/content/documentation/guides/quantization.md

@@ -144,6 +144,8 @@ The `quantization_config` can also be set on a per vector basis by specifying it
 
 To enable scalar quantization, you need to specify the quantization parameters in the `quantization_config` section of the collection configuration.
 
+When enabling scalar quantization on an existing collection, use a PATCH request or the corresponding `update_collection` method and omit the vector configuration, as it's already defined.
+
 ```http
 PUT /collections/{collection_name}
 {
@@ -331,6 +333,8 @@ In this case, you can set `always_ram` to `true` to store quantized vectors in R
 
 To enable binary quantization, you need to specify the quantization parameters in the `quantization_config` section of the collection configuration.
 
+When enabling binary quantization on an existing collection, use a PATCH request or the corresponding `update_collection` method and omit the vector configuration, as it's already defined.
+
 ```http
 PUT /collections/{collection_name}
 {
@@ -481,6 +485,8 @@ In this case, you can set `always_ram` to `true` to store quantized vectors in R
 
 To enable product quantization, you need to specify the quantization parameters in the `quantization_config` section of the collection configuration.
 
+When enabling product quantization on an existing collection, use a PATCH request or the corresponding `update_collection` method and omit the vector configuration, as it's already defined.
+
 ```http
 PUT /collections/{collection_name}
 {