Replace all occurrences of "nil" in IDatabase(Async) xmldoc with less ambiguous alternatives

[bdach]

Closes #2697.

All of the replacements were empirically tested to be correct via simple programs in combination with a local redis instance.

Notably, there is one worrying nit; in testing it turns out that the IDatabase.List{Left,Right}Pop(RedisKey, long, CommandFlags) overload which I talked about in the issue can actually return null, contrary to its nullability annotations. This occurs on missing key; in that case redis replies

Nil reply: if the key does not exist.

as per https://redis.io/docs/latest/commands/lpop/, which then at

StackExchange.Redis/src/StackExchange.Redis/ResultProcessor.cs

Lines 1546 to 1547 in cb8b20d

	case ResultType.Array:
	arr = result.GetItemsAsValues()!;

and later at

StackExchange.Redis/src/StackExchange.Redis/ExtensionMethods.cs

Lines 339 to 341 in cb8b20d

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	internal static T[]? ToArray<T>(in this RawResult result, Projection<RawResult, T> selector)
	=> result.IsNull ? null : result.GetItems().ToArray(selector);

turns into a null.

I briefly attempted to rectify this, but the RedisValueArrayProcessor poses a problem here, as changing it to derive
ResultProcessor<RedisValue[]?> causes the solution to light up in red, and I'd rather not mess with that as a first contribution without at least prior discussion concerning direction there.

[NickCraver]

This is great - went through and agree these are the best options. Thank you for this!