Improve docs for bystestring regarding null-termination

[nalimilan]

?bytestring says:

Base.bytestring(s)

Convert a string to a contiguous byte array representation
appropriate for passing it to C functions. The string will be
encoded as either ASCII or UTF-8.

It isn't clear whether the string will be null-terminated or not. I can fix this if you tell me whether it's the case or not.

(Also, it might be clearer to replace s with s::String.)

[JeffBezanson]

The underlying data will be a Vector{Uint8}, which always has an implicit zero byte after the end (it's put there by the runtime system, but you can't see it).

[stevengj]

In #7016 I updated the docs to mention that string data is internally NUL-terminated in Julia for passing to C.

[nalimilan]

@stevengj Yes, that's great, but I'd also like to add it to ?bytestring.