Previously, the docs implied that only Linux and Windows could use huge
pages.  That's not quite true: it's just that we only know how to
request them explicitly on those OSes.  Be more explicit about what
huge_pages really does and mention that some OSes may use huge pages
automatically.

Author: Thomas Munro and Catalin Iacob
Reviewed-By: Justin Pryzby, Peter Eisentraut
Discussion: https://postgr.es/m/CAEepm=3qzR-hfjepymohuC4XO5phxoSoipOjm6BEhnJHjNR+jg@mail.gmail.com