docs: explain the buffer archetype underscore convention #410

From the review result:

mutable_buffer_archetype_/ const_buffer_archetype_ -- what does the underscore mean? My take is that it's meant to be private and you use the aliases without underscore.

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026.

The guess is wrong, which proves the docs gap: the underscore types are public, and the non-underscore names are aliases that fall back to the plain buffer types on clang, where the archetype crashes the compiler. Nothing documents this. Either document the convention on both types, or move the underscore types into detail once the clang issue allows. (Overlaps #395; fold in there if preferred.)