Minor update to std.string module documentation. Added a brief comment t...

...o the introduction pointing readers to std.array and std.uni for additions string related functions.
>> @@ -8,6 +8,11 @@ $(D char[]), $(D wchar[]), or $(D dchar[]). The $(D
>> *_string) types
>>  are preferable because they don't exhibit undesired aliasing, thus
>>  making code more robust.
>>
>> +A number of common operations on strings are also found in the
>> +Unicode $(B std.uni), and array $(B std.array) modules. If the
>> +function you are looking for does not appear here, you may find
>> +it in one of those.
>
> The second sentence does sound like we're not sure where stuff is. I think
> the first sentence is enough.
>
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/D-Programming-Language/phobos/pull/2223/files#r14761351

Thank you for the suggestion, I think you are right. The second sentence
could be dropped.