docs: clarify meaning of HeaderValueOption.append (#12792)

Signed-off-by: Martin Matusiak <numerodix@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 831fb5089015fa97c7d7c4ad63e36ab7f593da61

envoy/config/core/v3/base.proto

@@ -293,7 +293,7 @@ message HeaderValueOption {
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
-  // existing values.
+  // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
 }
 

envoy/config/core/v4alpha/base.proto

@@ -285,7 +285,7 @@ message HeaderValueOption {
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
-  // existing values.
+  // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
 }