Move manual description before member list in reference documentation

[alandefreitas]

The manual description should come before the automatic, extensive list of members of a symbol.

That's what it looks like now:

https://94.beast2.prtest3.cppalliance.org/beast2/reference/boost/beast2/application.html

This is what cppreference does:

https://en.cppreference.com/w/cpp/container/inplace_vector.html