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)

Examinez la documentation de Boost.Algorithm et comparez ses entrées d’algorithmes avec la prise en charge de C++14 constexpr décrite dans l’issue. Le travail est terminé lorsque la documentation pertinente indique clairement quels algorithmes prennent en charge constexpr, sans laisser entendre une couverture non prise en charge.

Rédigé par le modèle d'indexation à partir du texte de l'issue.