[PATCH v3] mbstowcs: Document, test, and fix null pointer dst semantics (Bug 25219)

[Florian Weimer]

* Carlos O'Donell:

> On 6/1/20 12:35 PM, Florian Weimer wrote:
>> * Carlos O'Donell:
>> 
>>> How about this?
>>> ~~~
>>> This behaviour of accepting a null pointer for @var{wstring} is an @w{XPG4.2}
>>> extension that is not specified in @w{ISO C} and is optional in @w{POSIX}.
>>> ~~~
>> 
>> Looks good to me.
>  
> v3
> - Updated XPG4.2 text.
> - Fixed space after sizeof x 2
> - Fixed space after period x 1
>
> OK for master?

Yes please.