Clarify the documentation for IDatabase(Async).HashSet(Async) (#812)

The documentation for the overload of IDatabase(Async).HashSet(Async)
that takes multiple fields/values was somewhat ambiguous. It could have
been read to mean that fields not set as a part of the HashSetAsync are
deleted rather than preserved. The documentation for HMSET was clearer,
so this borrows some of the verbiage from the Redis documentation into
the documentation for HashSetAsync.

StackExchange.Redis/StackExchange/Redis/Interfaces/IDatabase.cs

@@ -329,7 +329,7 @@ public interface IDatabase : IRedis, IDatabaseAsync
         IEnumerable<HashEntry> HashScan(RedisKey key, RedisValue pattern = default(RedisValue), int pageSize = RedisBase.CursorUtils.DefaultPageSize, long cursor = RedisBase.CursorUtils.Origin, int pageOffset = 0, CommandFlags flags = CommandFlags.None);
 
         /// <summary>
-        /// Sets the specified fields to their respective values in the hash stored at key. This command overwrites any existing fields in the hash. If key does not exist, a new key holding a hash is created.
+        /// Sets the specified fields to their respective values in the hash stored at key. This command overwrites any specified fields that already exist in the hash, leaving other unspecified fields untouched. If key does not exist, a new key holding a hash is created.
         /// </summary>
         /// <param name="key">The key of the hash.</param>
         /// <param name="hashFields">The entries to set in the hash.</param>

StackExchange.Redis/StackExchange/Redis/Interfaces/IDatabaseAsync.cs

@@ -292,7 +292,7 @@ public interface IDatabaseAsync : IRedisAsync
         Task<long> HashLengthAsync(RedisKey key, CommandFlags flags = CommandFlags.None);
 
         /// <summary>
-        /// Sets the specified fields to their respective values in the hash stored at key. This command overwrites any existing fields in the hash. If key does not exist, a new key holding a hash is created.
+        /// Sets the specified fields to their respective values in the hash stored at key. This command overwrites any specified fields that already exist in the hash, leaving other unspecified fields untouched. If key does not exist, a new key holding a hash is created.
         /// </summary>
         /// <param name="key">The key of the hash.</param>
         /// <param name="hashFields">The entries to set in the hash.</param>