detail identifiers in public APIs could say __implementation_defined__

[vinniefalco]

For example here

template<class CharSet>
detail::pct_encoded_bnf<CharSet>
pct_encoded_bnf(
    CharSet const& cs,
    pct_encoded_str& t) noexcept;

Currently the workaround is to write

template<class CharSet>
#ifdef BOOST_URL_DOCS
__implementation_defined__
#else
detail::pct_encoded_bnf<CharSet>
#endif
pct_encoded_bnf(
    CharSet const& cs,
    pct_encoded_str& t) noexcept;