Doc request: indicate constexpr support for algorithms

Hi,
I believe most if not all of the algorithms in `boost` are `constexpr` if the compiler supports it and C++14 or greater is used. (yes?)

That's not indicated in the docs, as far as I can tell. I recently implemented a couple myself because I assumed the `boost` ones were not, from the docs; and at the time I didn't go look in the source code to see that they actually were. (yeah, I was being an idiot)

Revise a documentação do Boost.Algorithm e compare suas entradas de algoritmos com o suporte a C++14 constexpr descrito na issue. Considera-se concluído quando a documentação relevante indicar claramente quais algoritmos oferecem suporte a constexpr, sem implicar uma cobertura não suportada.

Escrita pelo modelo de indexação a partir do texto da issue.