Minor changes to reflect comments to PR

Remove type hasherFunc.
Adjust doc comments for the NewCustomHashPartitioner.

partitioner.go

@@ -82,16 +82,15 @@ func (p *roundRobinPartitioner) RequiresConsistency() bool {
 	return false
 }
 
-type hasherFunc func() hash.Hash32
-
 type hashPartitioner struct {
 	random Partitioner
 	hasher hash.Hash32
 }
 
-// NewCustomHashPartitioner is a wrapper around NewHashPartitioner,
-// allowing the use of custom hasher
-func NewCustomHashPartitioner(hasher hasherFunc) PartitionerConstructor {
+// NewCustomHashPartitioner is a wrapper around NewHashPartitioner, allowing the use of custom hasher.
+// The argument is a function providing the instance, implementing the hash.Hash32 interface. This is to ensure that
+// each partition dispatcher gets its own hasher, to avoid concurrency issues by sharing an instance.
+func NewCustomHashPartitioner(hasher func() hash.Hash32) PartitionerConstructor {
 	return func(topic string) Partitioner {
 		p := new(hashPartitioner)
 		p.random = NewRandomPartitioner(topic)