Add note to statistical facet docs about documents missing faceted field

Hey,

I was not sure how the computing of statistical facets worked in terms of how it treated documents that were missing the faceted field. I visited the #elasticsearch channel on irc.freenode.net, where @HonzaKral was very helpful with what happens - `"documents with missing fields are excluded"` - so I thought it would be a good idea to add a note to the docs and share this knowledge with the rest of the community in an accessible way :)

Cheers!