Doc request: indicate constexpr support for algorithms

[hadrielk]

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)