document Reset behavior as its non-obvious

index/index.go

@@ -122,9 +122,13 @@ type TermFieldDoc struct {
 	Vectors []*TermFieldVector
 }
 
+// Reset allows an already allocated TermFieldDoc to be reused
 func (tfd *TermFieldDoc) Reset() *TermFieldDoc {
+	// remember the []byte used for the ID
 	id := tfd.ID
+	// idiom to copy over from empty TermFieldDoc (0 allocations)
 	*tfd = TermFieldDoc{}
+	// reuse the []byte already allocated (and reset len to 0)
 	tfd.ID = id[:0]
 	return tfd
 }

search/search.go

@@ -88,9 +88,13 @@ func (dm *DocumentMatch) AddFieldValue(name string, value interface{}) {
 	dm.Fields[name] = valSlice
 }
 
+// Reset allows an already allocated DocumentMatch to be reused
 func (dm *DocumentMatch) Reset() *DocumentMatch {
+	// remember the []byte used for the IndexInternalID
 	indexInternalId := dm.IndexInternalID
+	// idiom to copy over from empty DocumentMatch (0 allocations)
 	*dm = DocumentMatch{}
+	// reuse the []byte already allocated (and reset len to 0)
 	dm.IndexInternalID = indexInternalId[:0]
 	return dm
 }