Add package doc comment

Also adds a TODO and improves clarity of internal package comments.

Author: wafer-bw

Diff for: memkv/internal/underlying/underlying.go

@@ -1,5 +1,9 @@
 // Package underlying provides the underlying data structures for the key-value
 // store.
+//
+// This makes it easy to extend out the data structures in the future and
+// enables easy testing of the store's underlying data via memkv.Store.Data
+// (see memkv_export_test.go).
 package underlying
 
 // Item is a wrapper around the instances of data to be stored allowing for

Diff for: memkv/memkv.go

@@ -1,3 +1,4 @@
+// Package memkv provides a generic in-memory key-value store.
 package memkv
 
 import (
@@ -8,6 +9,7 @@ import (
 
 // Store is a generic in-memory key-value store.
 type Store[K comparable, V any] struct {
+	// TODO: should this be part of the underlying data?
 	mu       *sync.RWMutex
 	capacity int
 	data     *underlying.Data[K, V]