Update string_view docs

[Lastique]

The header path is no longer correct:

core/doc/string_view.qbk

Line 15 in 12f5f51

[section Header <boost/core/string_view.hpp>]

Also, it would be a good idea to mention that boost::core::string_view is not intended to be used by users (hence it is in detail) and is supposed to be used by Boost library writers.

[glenfe]

I don't think string_view.qbk is processed (since it isn't included in core.qbk) so no HTML etc. is generated for it.

[pdimov]

string_view.qbk is no longer included. I just didn't delete it but I will if you insist.

[Lastique]

Sorry, I didn't notice the docs were not included.

I don't insist on removing the docs, just asking to make it actual. If you intend to make it available, even if for library maintainers only, it should point to the right headers and provide guidance to the reader. If you're not going to publish the docs, I don't see the point in keeping them, but I'm fine if you want to keep them for some reason.

[pdimov]

The future of string views is still unclear, so this documentation might still come into use, once we (Boost) figure out what we want and in which library and namespace we want it.

[Lastique]

Ok, feel free to close this if you don't intend to update the docs at the moment.